To be slightly serious about this for a moment, compared to the full-haired folks shown, it's a very small percentage of us people of scalp who have really been shown. And as Patrick Stewart once mentioned about there still being baldness in the future, "by the twenty fourth century, no one will care."
I do remember all the buzz (no pun intended) when TMP came out about Ilia. Months before I even saw the movie, I remember people talking about the new alien who was a bald woman. That was a lot of word of mouth publicity. Also, probably wasn't too expensive an alien to create. Not that they seemed to be watching the budget too much on that movie. But just shaving her head instead of expensive prosthetics may have saved enough money to give us ten more minutes of the Enterprise flying into Vger.
And don't forget, Walter Keonig is also a baldie, although I'm not exactly sure when that happened.