Yes, having a LOR from a dentist is perfectly okay. When I mean okay, I mean it's probably going to be an "okay" letter.
I don't understand how one would obtain an excellent LOR from a dentist you shadowed unless you bonded/clicked very well with the dentist, which I don't see happening much when shadowing. But then again, there are a number of schools that require a LOR from a dentist, so definitely check your school list. Otherwise, not having one is totally okay too, and I would personally find someone that would be able to write a better one like another professor or your job/volunteer's supervisor.
Best of luck~
