Yonko titles aren't official titles that are gifted to you after winning a competition. They are well-accepted rumors. And the fact that someone like Morgans, or anyone for that matter, would even entertain the idea that Luffy could be an emperor without being a top-tier clearly proves the title isn't about the captain having a top-tier power level.
Akainu didn't acknowledge it, Kaido didn't acknowledge it, Shanks didn't acknowledge it, Big Mom didn't acknowledge it, Blackbeard didn't acknowledge it, not a single person of relevance actually acknowledged it
Hell, even Oda himself didn't acknowledge it given that he still referred to them as the "Yonko", if Luffy was actually considered as such it would be "Goko"
Being a Yonko very much is about power, when we're first introduced to them Garp explicitly says they're the four most powerful pirates