Aren't Red Hair Pirates strong foes?
Aren't Big Mom Pirates strong foes?
Isn't Kaido a strong foe?
Fighting =/ being easily defeated
Fighting =/ attacking
His bounty matters because it should reflect his achievements. Less than 40 million in two years is nothing, he went against the emperors.
And let's not forget how ruthless the Kid pirates are against civilians.
We gonna ignore him tanking "Indra" and deflecting Kaido's "Gale of Destruction"?
Zoro did the second effortlessly. They are air slashes with no CoA, not a big deal. BM lightings are just lightings, they shouldn't be stronger than Enel's ones. And I'm not talking about his best moves. Everybody tanked those in Skypiea.
Killer being a veteran level character doesn't make him weak, it's extraordinary he reached such a level with no training and in just two years.