I can think of two possible reasons Wonder Momo never made it to the United States. One, it kinda sucks. Two, there's a crab fondling a woman's breast.
Since I can't think of any instances of concerned mothers protesting because they caught their children playing Superman 64, my vote goes to the crab. Add a few quick pantie shots and you've got a game that would have moms lined up outside with signs faster than lead paint on a toy uzi.
The titillation and gameplay are both pretty minimal. The entire game takes place on a stage, so there's nowhere to go really. Momo can jump, kick and kick while jumping. Most enemies go down with one kick. Those that require multiple kicks are usually kind enough to march mindlessly into kicking range over and over again.
The cruel truth is, most games with an element of sex to them completely fail when judged solely on the merits of the gameplay. And if a game is nothing more than walking back and forth kicking robots and aliens, there needs to be a lot more than a couple of quick glances of panties and a grabby crab to make up for it.