We have no evidence that Peter Gotti was a "dumbfella" from what I've read about him he was a strong boss and John and gene was mad because he wouldn't be their puppet

"McGurn likes you, so I make you. So you are now one of us, if you fuck up, we take it out on McGurn. He is your sponsor. Fuck up, it's his ass. You work in his crew, he is your capo."