Depends on the type of competence and incompetence than anything else.
Yeah, it's really not possible to answer this kind of question in the abstract. In some cases malicious intent will be more harmful, in other cases incompetence will be worse. It depends on a lot of factors.
I think that at the very least, a mentally competent US president will be smart enough to know when he can and can't rock the boat, and when he's pushing things too far. This is to be distinguished from someone (like Trump) who has never studied US history, has never read the Constitution, does not know the basic duties entrusted to the different branches of government, does not understand the law, has no background in public service, and is not capable of understanding the perspectives or feelings of other human beings. Other good examples of this personality type include Marjorie Taylor Greene and Darrell Brooks.
If you allow these people to flail around with unchecked power, they are just as likely to self-immolate as they are to immolate everyone else. Maybe I'll be proven wrong about this someday-- I hope not-- but I cannot see someone like Ron DeSantis or Glenn Youngkin ordering their supporters to attack the US capitol to overturn an election. I say this simply on the basis that they are capable of self-awareness, and they would understand the implications of their actions (which has never been true for Trump).
I think it is important for us to distinguish between Republicans with whom we have policy disagreements and Republicans who belong in padded cells. If you lump them all together, people will stop taking your argument seriously.
A mentally competent president will know how to go too far and gaslight everyone else into thinking otherwise. This cannot be forgotten when making decisions.
An incompetent one will just stumble along.
