To be totally honest, when I think of stuff that could be improve in mass effect, helmets are not one of them. Being honest here, but the whole thing about space helmets seems a wee bit nitpicky and manini compared to other glaring problems. Don't know why people care so much about helmets and holstering in this game, when the most it amounts to is one or two seconds of satisfaction. Thats just me though, I tend to be a bit more of a big picture sort of guy and really lenient on the smaller details if the narrative is satisfactory around those small moments of physics breaking. Obviously, not in the majority here, so apologies for toe stepping, just wanted to get a different opinion on here.
While Mass Effect can be considerably dodgy in the details department, and takes the laws of physics out for a night popping shrooms, there's something very basic about a space helmet and its necessity for dealing with inhospitable environments or the vacuum of space, even in fiction. No matter how whacked out everything is, no matter how much noise is depicted in the vacuum of space, things like tolerances to little to no atmosphere tend to be constant. Imagine if Liara was wearing that same breather mask on the moon. It may not serve for the bigger picture, but I would still consider it a humongous failure in design.
I wouldn't really consider giving characters entire new outfits an 'easy' to fix issue.
You've said this before, but your answers as to why this is are always vague. Considering that Mass Effect already recycles a few models of armor and clothing with different colors and patterns across the entire trilogy, save for some of the more unique designs introduced in ME2, what is so difficult about using this same practice when giving companions a wider variety of things to wear?