No, someone fighting in a catsuit and high heels is unrealistic. That is a fact, not opinion.
Now whether or not someone is able to suspend disbelief for that, like other unrealistic elements of the series such as the Lazarus Project or biotics, boils entirely down to personal preferences.
My bigger issue is internal consistency. It's difficult to argue realism.
Every combatant we face throughout the series is wearing armor, as are all of the allied troops in ME3. For Shepard to take people who aren't properly attired into hazardous situations makes Shepard look like every bit the doofus as those squadmates who don't choose proper combat attire.
Not to get into this debate too heavily (about to play ME2 actually,) but would it honestly surprise you? I'm not trying to mock the plot, but considering how much money and resources that Cerberus has stated to have, it wouldn't seem that far out of left field. The ability for them to create better catsuits makes more sense than the Lazarus Project imho, that's for sure.
There are some statements made in ME2 that Cerberus spent most of its resources building the SR-2 and bringing Shepard back. Apparently, they managed to get their hands on massive amounts of fresh funding in preparation for ME3. 
In any case, if Cerberus did have the ability to create better catsuits, wouldn't their resident assassin / ninja ballet master be using one? Kai Leng wore actual protective armor, with reinforced plates. Most Cerberus mooks wore very heavy armors - even the phantoms had fully protective, albeit lighter, armor.