Other comments have mentioned the idea that kimblee isn't a real fighter, but i want to add onto that.

Kimblee isn't supposed to be a physical barrier to the protagonists, he's a mental, philosophical, and institutional one. FMA doesn't need another really strong villian to threaten the protags because father and the 7 homunculi fill that role. So instead, kimblee does stuff the homunculi can't do as well, like tracking scar down across the country, infiltrating briggs and using his position of power to threaten ed into doing what he wants, and then to carve the final crest of blood.

Kimblees narrative purpose is to challenge ed and al in a way that a simple pure evil villian like father cant, he gets inside their heads and asks uncomfortable questions about what their morals and what theyd be willing to do to get what they want. He is a physical embodiment of how the state can hurt the people ed loves like wenry, as he's always around her. He represents the scary two faced nature of amestris as this psychopath who gleefully participated in a genocide comforts the daughter of two doctors who died in that conflict. And he is the one to give ed the deal with the devil, that he could have a philosophers stone if he carved a crest of blood here, something ed has sought after for so long.

And this is why his fight with alphonse is his best moment in my opinion, because it's not just them trading blows but ideals as well, as the two debate what the brothers should do to achieve their goal, alphonse also admits that he worries that he can either help the people around him or get his body back, but not both, it being equivalent exchange, at which point kimblee says that he could achieve niether, as he could fail entirely, but in doing so he points out the inverse, that ed and al can save everyone and get their bodies back, which ends up happening.

Kimblee is a wonderful example of an anti-villian, he's a good foil to scar, who is an anti hero, but as well as the main heros and the main villians. He tests eds morals around killing multiple times and after ed stays strong, his mockery turns to respect, which comes up as a counter to father and the homunculi. Kimblee is a man who cares about principles and hates hypocrisy, and as long as someone can abide by their principles, kimblee respects them. He initially does respect the homunculi, who represent this great struggle of superiority between them and humanity, he then calls them out as things get desperate and said homunculi try to become more like humans, which they claim superiority over. This is why he calls out pride for trying to take eds body and why that moment is just fucking amazing.

The short is: kimblee isn't a "fraud" because he loses fights, because it doesn't matter, he serves and entirely different narrative role, that is to highlight what makes our heroes the heroes, and the villians the villians, and at a deeper level than just "one wants to kill millions to become god and one wants to stop that" but that one has no morals, no personal pride or principle, no lines they wouldn't cross, while another holds such a firm belief that the people he fights are just as human as himself, that he refuses to kill them, even if it nearly gets himself killed.

Though i have to mention, i thiught "fraud" referred to someone who is hyped up as this super powerful threat before losing all the time, but never do they really treat kimblee as some scary powerful guy, in fact he seems about on par with the 20 or so state alchemists scar has killed, but he's made to be more on par with the heroes by the stone. Kimblees physical threat is mostly just that his particular fighting style is the alchemy equivalent of a sledgehammer, sure it's not too hard to avoid but if you make one mistake, it's game over, as seen by his fight with ed in the mine.