Given recent events in USA politics, I'd suggest not judging a (potential) candidate only based on his performance on an entertainment show, which is mostly scripted, with a writer team and a prompter to read from.

Colbert would certainly do better than Trump, but it would be good to set the bar higher, see how the candidates act under pressure which a challenging debate or interviewer. It's more telling than a friendly and scripted TV format, where the question are easy and obvious.