It's lazy and they did it to cut costs, it has nothing to do with art or how they thought she would look. Her hand for example is not even proportionate to what the Quarian's hands look like, it wasn't designed or thought out, it's a quick Photoshop edit to save money, the same kind of crap they did on Dragon Age 2.
Designing a character based on a real model is fine, I never complained when they modeled Miranda after the real actress, they put the work and effort into actually designing her in the game and making her a character, with Tali they found a photo online that they thought looked interesting, did a 30 minutes at best Photoshop edit on it (if it took them longer than 30 minutes then whomever does their graphics design needs to be fired), then said we're good to go. It's cheap and it's lazy, done to cut costs.
Yeah, and that doesnt surprise me really. I mean I doubt they ever bothered to model a face under the helmet, why would they? So faced with paying for concept art, then modellers, texture artists, maybe even animators if you want to have an option for her to have multiple expressions and so forth... i could see the worker-hours hitting thousands of dollars, maybe tens of, which does seem expensive for the purposes of putting a photo on a desk.
It's too bad though, as fully developing a suitless tali would open doors for later DLC that I think a lot of fans would dig.
Everybody good, plenty of slaves for my robot colony?