Power levels aren't everything. You can't reduce every villain in the story in that way.
Zemo won.
Zemo is a completely different kind of threat than a cosmic-level villain like Thanos or Galactus (which is also why he worked so well). He wasn't threatening the Earth or the universe, he was threatening the Avengers personally.
You're right, power levels aren't everything, and it's the villains who subvert those kinds of expectations that work best. But, unless Marvel took a radically different approach to Galactus than anyone expects, he would be more akin to another Thanos than another Zemo, and I don't know if that really makes for a more interesting bad guy or a more believable struggle.