I don't think its necessarily belief, but I don't think there's any truth to the superstition about not being "supposed" to look into mirrors in dreams.

I have looked in mirrors in dreams, and I've never gotten a reflection that looks like me. There were a few times I was deformed, and more often than anything else I'm just another person with a completely different appearance. I don't think it has to have anything to do with belief, either though, because I dont remember ever hearing that rumor before now. I think its a lot like when you look down at your hands in dreams, and you have the wrong number of fingers or they seem to melt away. Or when you look at a clock in a dream and the time keeps changing. Its just because there's a level of realism that isn't present in dreams, and perhaps sometimes its also symbolic.

however, the power of suggestion can be a powerful thing, so if you hear something like that, I think its more likely that it might pop up in a dream of your own.