Is delusion a mental illness?

When Biden did poorly in the first debate, most Democrats admitted it. This time Trump did poorly (even by FOX NEWS) and his base along with Trump himself, refuse to face it or blame everyone else. Do we really want a totally delusional person with his finger on the button regardless of his policies?

“When a prizefighter loses a fight, the first words out of his mouth are, ‘I WANT A REMATCH,'” Trump posted on Truth Social, asserting that he won Tuesday’s debate with Harris despite some polls showing otherwise.