It's not neccesarily just the bare back that's the issue. The rest of her costume is sooo out-of-character. It looks like a bunch of drunken frat boys took scissors to it-cutting a "Power Girl" breast window, the open middrift, the sorta-but-not quite bare thighs, plus the back, etc.

Again, the look's not uncommon in comics, just not Sue Richards's persona, which has been well-established for 50 years of publishing.. That doesn't mean she has to be humorless, or that she should wear 'mom jeans'. Byrne and Waid handled her the best imho, juggling her responsibilties as a superheroine, a mom and a loving wife.