I always believed Marvel to have the, what i like to call, the mystique/shang tsung effect. Just because she/he can morph into someone and have their "powers" doesnt make him/her as good or as powerful as the original person. Yeah Marvel may have the power of Zeus, but the real Zeus is still much more powerful. etc. i believe. I never read anything with Marvel cause i think he looks like a fruit basket and he never interested me (im more of a Black Adam guy), so i could be wrong. Anyone wanna clarify.
Besides Thor has control over Marvel's main power source, lightning. True its mystical lightning from Shazam, but despite what people say i still believe Thor has some influence over any kind of lightning, natural or magical, just because he's a god (god's usually delve into echantment and magic which i think is what seperates them from any other "strong race" of beings) and for the simple fact that its still lightninig regardless (just with magical properties) and Thor is god of the storm. Im sure someone will refute this but watever, im sticking to this theory.