Well yes, but you have to be careful...
I don't think there is a good defined line here, so there is a tendency for questions asking whether or not something is a bug to get closed as a bug report regardless.
(I have seen way too many questions closed as "bug" when there is no bug anywhere.
Partially because I have seen so many bad bug CV uses, I'm predisposed to not close something as a bug until it is very clear that it is really a bug. sadly many other users, will vote as a bug as soon as it sounds like blender is not doing something like expected.)
However, this type of question is something that could be very useful. For example when asking about how a tool should function, or is ______ normal, yes I would say ask.
Here is an example:
Why does the grab tool (G) have lines and only move on edges? Is this some kind of bug? It only happens sometimes.
Tell me in the comments below what you would do with a question like that. (wow that sounds like something from a youtube video)
Ok, for one the question is unclear (like many that are insta-closed as bug) but it sounds like blender is messing up. We all know that pressing G should let you move anything anywhere.
Yet this is not a bug, but rather a (possibly a) new user not knowing what double pressing G will do. That OP entered edge slide
without realizing it. Thats all (in my fictional example) no bug, everything is working as it is meant to, just a new user not knowing what is really going on.
Is that type of question allowed? definitely. (Just please ask a better question and make it as clear as possible.)
Would that type of question get closed? probably. If not as a bug (incorrect reason) then as unclear, at least until somebody edits and makes it better.
I would prefer if the title said "Is it allowed to ask if a behavior is normal?" Then it would be a 100% yes, but for some reason you stick "bug" in a question and that changes how people see it.