There is this stereotype of thinking when it comes to dreams that the kind of person you are "psychologically" when you sleep is different from the kind of person you are when you're awake (invoked in conversations such as whether hearing a voice when falling asleep is the same as hearing a voice vividly enough while awake that someone might be schizophrenic). So then the reasoning often goes that if you look in a mirror, you always see the version of you that you are according to the laws of how the expectations work.

The last time this happened, what did you see?

For some reason my dreams always seem to lack details visually. Or at least I always remember them like that. I have no idea why, my eyesight is actually pretty good when awake.

So if I see a mirror in a dream, I just see what I assume to be my reflection, but I never see it clearly enough to make out any details, and the parts I do see aren't notably different from how I actually look.

I am however not always myself in my dreams. If I watched a movie or series or played a game right before bed then I frequently dream in character of a memorable character of that movie/series/game. But I cannot recall having come across a mirror in one of those "in character" dreams.

I think you are correct in that you should see what you expect though. I occasionally have lucid dreams, and then it really feels like whatever you expect to happen, happens. In theory that makes the dream controllable as if you change your expectations the dream changes. However getting full control over your expectations is in my experience a lot harder than it sounds.
I tend to often doubt myself so if I try to steer a dream that way, a part of me always expects it to go wrong, so it always does.
That's not related to mirrors, but I guess I still is another way dreams can tell you something about yourself.

Myself with a bad haircut I didn't remember getting.

I'm seldom conscious of my body in dreams, but when it happens it's often a low-key body horror to which I'm strangely blasé. Last dream with a mirror view, my face had a hideous trypophobia condition that should have been a lot more painful. Oh, that's happening.

Do you know the baalspawn monster form from BG2? Something like that.

That was a joke.. I have incredibly detailed dreams, meaning that there are quite a few I remember vividly years later, but no, cannot actually recall looking into a mirror in any of them.
(though in steamy dreams the parts of my body in my field of view were similar to real life)

There is a movie I supposedly saw as a 9-10 year old child, but I'm no longer sure it was not a dream. I remember it in vivid detail, but there were only so many dinosaur movies made in the 70-80s & it does not seem to be one of them. Described the plot & prominent scenes in the past on movie subreddits & also to LLMs & there is no hit..