Starfleet is only a military organization when the mission requires it. And from what I know of my military friends, yes, acting like a juvenile idiot is sometimes exactly what happens. It's not like they do it while they're in the middle of a crisis. Voyager isn't just their place of work; it's their home.
Just because Star Fleet isn't a military organization doesn't mean its not an organization that doesn't demand professionalism.
If some alien diplomat was visiting and some star Fleet officer thought it would funny to throw a drink in his face, I would think that person would be punished for such behavior. Also if some ensign decided to wear an "I'm with stupid t-shirt" I think he would he sent back to quarters to change.
There times where Tuvok comes across as only the sane person on the ship. Harry Kim is the worse offender, he really comes off as a simpering man child. He never really developed into a good seasoned officer, he managed to stay a green ensign for 7 years. Frankly there should been episode where he proves himself, saves the ship and gets a promotion, that would have shown more character growth then him remaining a green ensign for 7 years.