You cannot forget that the marshmallow man was in fact an elder god, ZOOL!
And the statue was held together by goo, powered by the goodwill, and barely managed to beat up a building!
Okay, the god was named Gozer. Zool was "The Gatekeeper" that possesed Dana and turned into the demon-dog.
And for the record, the mood-slime kicks ASS!
Anyway, I figure the fight would play oput something like this. The Statue would have an initial advantage, cause the torch (which bursts into actual flame when slime-ified). However, it's slow moving nature would lead Mr. Stay-Puft to a victory. However, Puffy would be weakened by the fight, and would be finished off by sugar-high children.