I notice that people don't mind much the fact that Kidd in this arc has actually been fighting with only one arm, mostly because he is able to somehow substitute the other with a mechanical one, but to me it's clear that he has been significantly nerfed.
Unlike Franky, who turned his body into machine as a means to strengthen it, Kidd is a devil fruit user, and not a cyborg, meaning that Kidd's mechanical arm is basically just his version of Perospero's fake arm that he created with his devil fruit power.
And this means that, just like Perospero, Kidd has to focus part of his devil fruit power just to maintain his mechanical arm in movement, rather than being able to just move it freely, because it's not part of his body, so he lost all the muscular power he had in the real one, and it's also harder for him to use the mechanical arm as a point of focus to attract metals around around him and create some giant arm, for example.
I think this was made clear by the fact that when he used Punk Gibson he covered his real arm with metal until forming that giant one, and the impact the attack had was powered by by the muscles of his arm, meaning that it was much stronger than it would be if he had used Punk Gibson with the mechanical arm.
So my conclusion is that Kidd should have had much more impressive feats in his fights with Kaidou and Big Mom if he had two arms.
But nerfing him by making him lose one arm was the means that Oda found to make Kidd an equal to Law and inferior to Luffy.
Especially in the case of the Straw Hats, the bounties are far from being accurate indicators of power levels, but in the case of the Worst Generation, I'm quite sure it's fair to assume that the difference of number between the bounties of Kidd, Luffy, Law, Hawkins, etc. (except for Zoro) was more or less a ranking of power between them, because Oda simply had no reason to make a new pirate character that was weaker have a higher bounty than a stronger one that was being introduced in the series at the same time.
So Kidd was originally portrayed as equal or a bit stronger than Luffy, and significantly stronger than Law.
But Oda wanted Luffy to be the one to beat Kaidou in the end, while Kidd and Law would help each other to beat the other Yonkou, and this wouldn't make much sense if Kidd were still equal or stronger than Luffy, so Oda solved the problem by nerfing him with one arm less, so that he would fall to Law's level.
