I don't think it was a good performance by Trump but it was hardly a disaster and something that would have taken many people by surprise. You know what you're getting with him - he's mad and is going to bend the truth and go off on tangents. I expect he'll lose the election, but that debate won't have lost it for him.
Harris is the beneficiary of a late change of candidate and lack of scrutiny as a result of the change.
Neither of them are any good and it's terribly sad for the US that one of them will become President.