Cthulhu, in the original story "The Call of Cthulhu", is not very impressive. Its plot ends with it
being defeated by being ran over by a motorboat
and the general moral of the story seems to be that
while the evil, ancient being can be defeated, the foreigners, minorities and people of other races and cultures will always be out there to get us in horrifying ways we cannot imagine.
It's feeble (by the old gods standards), it's simplistic and I don't think it's even all that popular; I was never a huge Lovecraft afficionado, but I have an impression that some other gods, like Shub-Niggurath, Hastur or Nyarlathotep were used more often. So why was it chosen to represent the entire mythos? Surely one of the others would be more representative?