The criteria to be a Yonko is to be the leader of:
- an empire
- a strong crew/fleet
The territory is obviously there.
This also means that Mihawk and Crocodile is collectively powerful enough to pass the benchmark of how strong a collective Yonko crew is needed to be.
Even Law outright says this: "If Buggy has those two under him, yeah it makes sense he's a Yonko".
I think this makes sense. A top tier (that too amongst the strongest top tiers) is enough to solo multiple standard commanders. The said crew will be weaker of course if people knew Buggy was fodder, but still it's about comparable to a Yonko crew without a (proper) Yonko as well.
Also note that Marco was considered a Yonko candidate as well (inspite of leading a Yonko crew without it's proper Yonko - WB), and he is roughly comparable to King in power. Meaning, top tier power level is not a strict requirement based on precedence as well.
So, Buggy would've had the title regardless.
But if the question is, would CG be seen as a threat comparable to other Yonko crews if they knew Buggy was fodder, instead of Shanks level?
Of course not. The world thinks CG has two characters who are comparable to Shanks in power, the difference between perception and reality is 1 Shanks.