Is it common to ask professors for an editable copy of their lecture notes (so I can improve them)?
No, in my experience this is not common.
But that shouldn't stop you from asking. Your suggestion is certainly within the reasonable parameters in which simply offering your help wouldn't be offensive. (Provided of course you do so tactfully, i.e. following the usual etiquette when pointing out that someone else's work could be improved upon.)
I have friends who would be very happy to receive such an offer as they would finally turn their messy notes into something of high quality! But bear in mind that while accepting your offer may seem like a "no brainer" to you, there might be perfectly reasonable reasons why your professor might turn you down. For example, even though your offer seems like a way to improve the lecture notes at no cost to the professor that is not the case. Improving the notes will require co-operation and further editing by your professor as your professor will have to make sure that you did not introduce significant mistakes or misinterpretations. And they simply might not have the time for that!
There are two parties involved in this, you and your professor. Let us think from both your perspectives.
You wish to help your professor and also want to ensure that the notes are of the best possible quality ( you think they are not currently).
Your professor depending on his objective and personality may like this idea or frown upon it. If the lecture notes are a property of the university and the professor is forbidden from using it in any other context without their permission then he should be more open to sharing the source with you. But if suppose he was planning to write a book or thinking of leaving his job and working in some other place he might be reluctant in sharing it with you as he would like to build upon it, later.
If he were having a personality who does not like criticism from his students or for that matter anyone he would naturally get offended by your proposal. Also, it depends on how much he trusts you in sharing the source with you, not only on how good you will hold on your promise of not sharing it with anyone but also on your ability to improve the notes.
From your perspective, you are a good person offering help for free for the greater good. This is commendable and you must politely approach him. If you are really keen on doing it, you could write the notes from scratch for one lesson basing on existing notes to show that you could add value. Then send the notes to the professor asking for his review and request him for the latex source as it will help you in working efficiently.
Professors in many universities ask the students to share their notes and the best-written notes get extra credits. So students engaging in building material for lectures is not uncommon.
Show the value of your proposal
Drop a mail on these lines (omitted salutations and format that is likely to be region specific).
Your notes were helpful for me but I did notice that they could be enriched by additional material and minor formatting improvements, for example as in the attachment. I felt these changes would benefit future students. I would be happy to make such changes and submit for your approval. If this proposal sounds fine, you may please suggest how to take it forward.
Showcase the value you plan to add in the attached document. This would :
Make your intention and the value clear.
Give the professor, the option to respond keeping his obligations with respect to ownership of notes or any other concerns.