The way I see it, the twins were new to the 'game'. Least we forget that in the MCU Iron Man, Thor, Cap - Widow (etc etc) they have been in the game a long time. Take Widow - Her get-up in IM2 to how it was in AoU huge difference.
I think it would have been silly if from the get-go two young hydra experiential subjects had been kitted out with costumes that gave the impression they had been in the team as long as the others. The fact they are impromptu, basically civilian clothing was for me the right choice especially with the late change of side switch.
Scarlet Witch has long been a favourite of mine, but I have no real issue with the MCU adaptation as it is early days and there is plenty of time for the character to develop.
In terms of costume: For me the teaser at the end of AoU shows that it is a work in progress as one would expect, and we WILL get a more complete and mature get-up.