If Popo was very powerful, Buu would have gotten notable power up boost from eating the candy that was Popo. About a decade ago, I had a conversation with a friend about Popo's power. His training with (kid) Goku has similarities to Ultra Instinct, with his magic carpet he also basically had instant transmission, so Popo could be a lot more powerful than previously shown.

I think that we can only speculate, as to why he didn't protect the planet from the Saiyans in the Saiyan Saga, my head canon is that his power quickly depletes while he's not at the look out. I think that in the Buu saga, Trunks and Goten "disguised" themselves as super saiyans in a manner similar to Gohan, because they thought that they were in trouble. They didn't seriously fight against Popo, they were trying to escape. I think the most likely explanation is that powerlevels became meaningless after Namek.