The idea that mutant powers don’t work quite right fits well with Marvel’s storytelling style. Many Marvel heroes have powers that cause more problems than they solve, and in the case of mutants, many just plain don’t work the way they should.
It’s when the mutant in question doesn’t fit in the way we expect that a character might be somewhat short-lived as an active member of the team. That brings us to Dr. Cecilia Reyes.

Cecilia Reyes was brought onto the team at a time when, once again, someone or something was out hunting mutants and some X-Man who was lucky to still be free had to bring in whatever mutants he or she could find to save the day. In this case, the crisis was Operation: Zero Tolerance where the madman Bastion (who, well, wasn’t human anyway) had sent Prime Sentinels out to bring in or bring down as many mutants as possible. Prime Sentinels were disguised as human beings in part because they somewhat were people with cybernetic enhancements that made them adapt to any and all mutant powers while out doing whatever it was they were doing. The free X-Man in question was Iceman, and the other mutants were more or less the Israeli superheroine Sabra and the Morlock Marrow. Then there was Cecilia Reyes.
What we her deal? Well, she was a medical doctor. Those are generally handy to have around. She also could generate a forcefield of sorts. It was subconscious, but whenever something attacked Cecilia, the forcefield would kick in and protect her from all physical harm.

The key words there is “physical harm”. Though Cecilia’s shields would protect her body, she still somehow felt the pain of every impact. So, yeah, mental or psychological pain was a whole different ball game. And since she couldn’t exactly control the power at first, it just kept her alive but if you still felt the pain of an impact, would you want to use the shields all that often?

Of course, most X-Men join the team and then pick a codename that goes somewhat along with their powers or personality or general appearance. For example, the aforementioned Marrow took her name (probably her only name since she was a Morlock) due to her body constantly growing bony protuberances that she could use for armor or disposable weapons. And since she is a Morlock, that means any powers she has sounds awful anyway. But most any mutant, whether prominent or Misplaced Heroes like Beak or Maggot take superhero names that reflect their powers in most cases.
But not Cecilia Reyes! She thought codenames and costumes and the like were, well, silly, when all she really wanted to be was a doctor. To be fair, codenames and costumes are incredibly silly, but they don’t seem to be within the world Reyes lived in. If some dude dropped down in front of you or me and proclaimed himself Batman while wearing a tight costume with some bat-insignias on it, I don’t think we’d be quaking in fear so much as laughing our asses off. But in the Marvel universe, this is normal. So, while it is admirable for Dr. Reyes to work on keeping her medical career going, let’s face it: the universe probably wouldn’t let her.

So, Reyes more or less hung out with the X-Men as a medic for a while, and then she left and mostly comes back when they need a doctor. She still thinks superhero stuff is silly, but if it’s dangerous being a mutant on the street anyway, you might as well hang out with the cool kids with the laser vision and healing factors. Then maybe you won’t need to use your so-so forcefield power quite so much.
It’s not the boobs that are wrong, it’s the costume. I know of no fabric save experimental spray-on stuff that behaves like that. And that’s not shiny.