At one point, one of Biff's goons makes fun of Marty for wearing a "life preserver." It's basically a down vest, and many people would indeed wear them in the future… but it's not like everyone in 1985 was wearing them. Could there be some other reason the costuming people and director decided to make Marty a puffy-vest sort of guy?

Yes indeed. In all his interactions with his parents, Marty is trying to preserve his life.

"Dork thinks he's gonna drown," say the bullies. Well, yeah, in a sense: the slow erasure that Marty experiences when it looks like his parents aren't going to ever get it on is kind of like drowning. It's slow, it's painful, and it's a bit like sinking below the surface.