Are nightmares considered a good idea for a novel?

I don't know what kind of nightmares you have; admittedly, I very rarely have nightmares that I remember the next morning but the dreams I remember for a short while generally don't make much sense (having their particular "dream logic").

I doubt that a nightmare could carry the plot for a novel.

That said nightmares might provide you with some ideas, themes or plot elements around which you can build your novel so I won't say they are necessarily useless.

Nothing is 'a good idea for a novel' per se. It becomes good only when it's being convincingly worked into a novel written by a competent and imaginative writer.

I used a nightmare for a short story once, which did quite well, so why not?

Depends on the nightmare. If you've had a nightmare you think would make a good novel, then why not write it?