How can Buggy only have 3.1 , and if Mihawk has 3,6 for strength why tf should he hide behind Buggy’s clown circus ??
Okay, I'll give you a serious answer.
First of all. It appears to me that you are mixing two very different arguments:
One argument is: The WG's perception of Mihawk and Buggy.
A different argument is: Mihawk's actual strength.
First topic:
The WG are entirely mistaken about Buggy, they believe he's Shanks' peer, someone who knew Whitebeard personally, and a genius who has created a whole system that turns civilians into marines', placing them in checkmate harder than any other Yonko has ever before.
They also believe Mihawk and Crocodile have chosen to follow Buggy.
They gave him a 3.1B bounty for all of that.
They still gave a much bigger bounty to Mihawk despite believing he's now Buggy's follower. Why? Because he killed some marines a long time ago? That probably did boost his bounty somewhat in the past, you can't say for sure he was some psycho. What we can say is that if marines hated him so much for being a marine killer, they would have captured or killed him, but that never happened. If he wasn't monstruously strong, why make him a Shichibukai instead of teaching him a lesson?
Despite being, in the Marine's eyes, Buggy's executive, he's still seen as a much bigger menace than Buggy individually. Without resorting to headcanons, the easier answer as to "why" is simple: strength.
They gave him a 3.6B bounty, above his Yonko captain, for that simple reason.
Second topic:
Mihawk's actual strength. I have addressed this partly int he previous point, but there's a bit more to say. You have asked "why tf should Mihawk hide behind Buggy's clown circus".
Mihawk gave his answer himself. He's "hiding" behind Buggy so that "Buggy takes the heat". Mihawk longs for a worthy challenger (as we know both from his interactions with Zoro and the vivre cards), but fodder marines coming one after the other for his head isn't a challenge, it's a nuisance to his otherwise peaceful, disenchanted life. So he said so himself "I don't care about the Yonko title, lets let Buggy take the heat, we can take him out any time we want if he becomes a nuissance".
Problem is... he miscalculated, because Buggy has now rallied Cross Guild into a much different objective, and he and Crocodile will be forced to take part in the PK race one way of another or they will lose the crew (or most of it). It's an obvious humoristic way for Oda to throw him into the conflict without messing up his personality and intentions.
It's also likely part of what actually moves Croco's and Mihawk's hand in this matter is that the OP can really help them build Utopia, but it's certainly not how they wanted to go about it.