Well, she wasn't created as a solo headliner. None of the ladies of that era were (the others being Wasp - who if she had a solo would probably be called Winsome Wasp, even before Nadia showed up - and Marvel Girl, who nowadays doesn't have a codename and is just Jean Grey). And the name Invisible Girl was a pun, as she's a distaff counterpart to the literary Invisible Man.