If your super power involves zipping along at extremely high rates of speed without the benefit of a canopy or windshield, leaving your nipples exposed is a Very Bad Idea::
And so the pernicious influence of Joel Schumacher on super-hero costume design claims another victim. Well, two victims, namely the severely wind-chapped nipples of Mr. Rocket Racer. That's probably why he looks so grumpy. Either that, or he really has to pee, and in his enthusiasm over cutting out nipple-holes he forgot to carve out a potty portal.
(Image and character ©Marvel Comics.)