The folks over at TheHanafudaTimes.com sure seem to think so:
"Despite being absolutely ridiculous, this game is actually one of WiiWare’s more entertaining titles: Not something I can say for most games priced at 500 points," writes reviewer prettymanfairy. "If you’re into the WTF genre, don’t let this game get away."
Having spent some quality time with Muscle March, I have to agree--both that the game is entertaining (and ridiculous) and that it fits squarely into the "WTF genre," as prettymanfairy so creatively calls it.
Does that mean it's worth 500 Nintendo points ($5)? I think so--if you're a fan of other WTF-esque games like Katamari Damacy and WarioWare: Smooth Moves.
One more thing to think about before you hit the "buy" button: Muscle March is fun, but the fun is over fast. I'm OK with that, as I have the attention span of a goldfish, but if you're a more dedicated gamer you may feel a bit ripped off.
See also: "Monday, Jan. 18 = the gayest day ever for American Wii owners" and "The best (and probably gayest) ESRB ratings summary ever"