It has the same meaning as "mi sembra ..." o "credo ..." with a somewhat bigger emphasis on the fact that you're convinced you're right (this depends on the intonation though!). However, it is a very colloquial expression, never use it while writing!

In this context, "mi sembra ..." can be translated as "I think/I believe it is ... " Mi sembra un po' troppo giovanile = I think/believe it is a bit too youthful The slight difference in meaning between the two forms only relies in the fact that "sembrare" (to seem) implies appearance (in this case visual appearance), and thus it is understood that you can see the dress (or whatever you were talking about, though I'd guess you were talking about a dress here) in front of you while you are talking. The sentence with "to think" (pensare) or "to believe" (credere/ritenere) is more general.