Can a good but unremarkable PhD student become an accomplished professor?

Yes of course.

Some PhD’s are really good at research, or they just prefer research.

Others however, like to pass on knowledge and when a student « gets » something they are pleased for them.

So being able to explain concepts to others and remembering or understanding why you found certain concepts difficult and using that experience to help other students can be very rewarding.

The downside is it comes with things like grading... But having clear marking schemes helps with that, both when you mark and when students want feedback.

Some people are adept at helping other people understand and realize study material and also have new ideas even though them themselves maybe don't have so many "original ideas". There is a big need for teaching professors as well as research professors. Sadly these days seems many professors end up being neither.. just a vessel for acquiring grants and doing administration...

Depends on what your definition of a 'Professor' is. There are some professors who got to their position based on governance, teaching or/and research. In terms of research, I know of big name professors who operate more like a business person, so an outsider would think they are excellent researchers.