in cases like Thor and the Warriors Three it makes it easier to identify them. I recall watching a short introduction to anime years ago and it stated the reason some anime characters in the past (and present to a degree) had outrageous hairdos or costumes was to make it easier to tell them apart.
As for Asgard, I think as writers got more and more away from the "sci-fi, super-advanced Asgard" you got simpler designs. They are meant to look more Vikingish. The less eccentric costumes I suppose are to let the Asgardians be taken more seriously. I seem to recall some issues where Odin would go through two or three outfits in one issue.