How common is it to outsource tedious research tasks to undergrads?

I am a graduate student, and I often hire undergraduate research assistants.

My primary motivation in hiring undergraduate research assistants is to create opportunities for these students to get a sense of what research is about, gain some experience that can help them get a job or admission to graduate school, and help them figure out what they want to do next.

Sometimes my undergraduate students do work that is tedious, because research can sometimes be tedious. This is especially true when they are inexperienced, because it can be hard to see the bigger picture behind what you're doing. (They may not even have the necessary background to understand the bigger picture, at least not until they've taken some more advanced coursework and read a few dozen research articles.) A good supervisor tries to help students see the big picture, but it's hard.

Supervising undergraduate students is a lot of work for me. I don't do it because it somehow helps me progress in my research by offloading tasks that I really need to get done (if anything, it slows down my research). Usually the work I give to undergrads is work that isn't on the critical path for my own personal research, because I can't trust that it will get done quickly/correctly. And in most cases, by the time my undergrads have learned enough to really be useful to me, they're graduating.

I hire these students because I remember how meaningful my undergraduate research experience was to me, and I want to pay it forward.

To directly answer your question,

How ethical or common is it for grad students to actively hire undergrads into their lab for the sole purpose of outsourcing tedious labour?

It's probably much less common than you think, for the reasons described above. It can happen sometimes in the context of a research effort in which someone has to collect a lot of data, and it doesn't require any knowledge or skill to do so. But even then, it's often quicker for the graduate student to do it himself.

When it does happen, I see no ethical problem as long as the nature of the work is disclosed to the undergrad before he/she accepts the job. Compared to many other student jobs (working the phones and calling alumni for donations, for example), being a pipette monkey is actually probably one of the better jobs.

It is very common in my field (structural engineering) especially if the research involves testing of multiple material specimens (or samples), casting and curing concrete, preparing and installing sensors (strain gauges, LVDTs). We do it for many reasons,

1. "If you do not see it or touch it (get your hands dirty), you won't understand it." You can't expect a structural engineering student not to be able to work with construction materials! Especially if this student is looking for going to graduate school.
2. It cuts down on the time needed from graduate students to do "basic" labor work. Although this can be a double edge sword, the undergraduate student can miss up! We make sure that there is a graduate student or lab technician around the undergraduate students to supervise them and provide direction.
3. We only use the top 1-3 undergraduate students. Those who we know are interested in going to graduate school. This has many advantages as you can expect.
4. They don't work for free! We pay them too (since generally we don't include them in papers, maybe acknowledge them), this motivates them in a way.
5. They don't do all the "dirty" work, the graduate student still has to do his part!

I have a fair amount of experience with this - this was how I originally got into research, as an undergraduate, I've been the lab manager for a lab with a large undergraduate component in it, and as a professor I often include undergraduate research assistants in my budget.

Lets address the parts of your question one at a time, paraphrasing a bit for the sake of clarity:

Is it common?

Yes, it's reasonably common to hire an undergraduate assistant to handle boring tasks.

Is it ethical?

This line of questioning seems to have become popular on this site of late: "There is a thing I don't like, is it ethical?" without really articulating why it wouldn't be ethical. Lets say I hire a undergraduate to do the most boring job in my lab. Is that less ethical than if I was the manager at the local Arby's? Or the university library?

There is no ethical obligation to keep an undergraduate entertained and engaged. I have exchanged money for their time and effort. That's all.

Is this frowned upon and is it really any different than a typical undergrad research opportunity.

It's certainly not frowned upon, see above. As to whether or not it's different from a typical undergrad research opportunity, that very much depends on what you mean by "typical".

But I will say this: Research is often tedious.

Undergraduate research is often especially tedious because it's often a small chunk of a larger process, and one that can be "safely" given to someone with relatively little expertise and experience. That is not to say it is not essential work - what's given to undergrads is often "Someone has to do this, and it will take lots of time." If a graduate student or professor does it, they can't be doing other things. If an undergrad does it, they can work on advancing the project in other ways in parallel.

Consider some examples from my field:

• Abstracting Data: Much of the data for outbreaks of infectious diseases doesn't come in nice, machine readable forms. Instead, it comes in nice, human readable weekly reports, describing what happened that week. Wading through those is tedious and boring - find a number, update some rows on a spreadsheet, rinse, repeat. But someone has to do it. Similarly, one of my first research jobs was taking meteorological reports that were in old hard copies and transcribing them.
• Citation tracking: Find a number in the published literature - see where they got it from. See where they got it from got it from. Follow that rabbit hole as far as it goes. Again, this is pretty mindless.

Yes, these are boring. They also have to be done. And in my experience, they're valuable research experience. Wading through references looking for something is how I ended up writing my first review paper. A student (admittedly a graduate student) in a lab I worked in ended up being quite prominent for maintaining some data that was largely just tedious abstraction and making it public.

As @ff524 has said however, this is not without effort, especially if you're also trying to make sure the student is learning and engaging in research. Undergraduates are likely slower, more prone to make mistakes, and need more supervision. It's a lot of investment in time and energy for someone who - if all goes according to plan - won't actually be around for very long.