i'll admit that a lot of med students, including myself at times, act immaturely. however, there's a difference between being immature at lecture or socially and being immature with patient responsibilities. lots of my classmates do the former, but i've only rarely seen the latter. i guess i'm trying to emphasize that people who don't seem to take life too seriously or themselves too seriously may still take medicine and their jobs seriously.... and i think that's what counts.
in general, i'd discourage people from making judgements about others' attitudes and appropriateness/inappropriateness for medicine. you could argue that goofing off when not with patients keeps you sane and allows you to cope with the gravity and responsibility of learning medicine. you could argue that being serious in your personal life reflects that attitude necessary to be a responsible doctor. in the end it depends what works for you; both groups are usually responsible on the patient floors, and both groups do just fine as doctors.