It's because of a few things, firstly Simpsons was at the end of Season 3/early Season 4 when the game was being developed so Radioactive Man didn't appear much. There was an episode with the character at Season 2, probably THE episode that Acclaim suggested to make an episode out of. Unlike later on with the comic spinoff and appearances, there wasn't as much character design artwork done at the time. This is what the character looked like in the episode. Yet...

Whoever was the graphics artist wasn't really good at doing these sprites even with the limitations (the enemies at least don't look too bad). Imagineering always seem to have a certain look to their games. Also they don't seem to emulate the Simpsons style very well... Bart looks very off in their games and he's the main character. When the likes of Konami (especially considering that Japan didn't get the series until 1992), Ocean, Arc Developments, Audiogenic and Sculptured Software all did a better job at the sprites. For some reason Acclaim kept giving Simpsons games to Imagineering on Nintendo consoles until the SNES... In the Game Gear version (done by Tweeny Weeny Games), Radioactive Man looks a lot more like the character.

Because he's radioactive.

Why does he look like that?