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.