Listened to the book and watched the extended cut just the other day. It's quite funny, definitely part of Andy Weir's charm, but it's far more dramatic a story than it is a comedy.
Some of the longer, reflective scenes in the extended cut give it a more somber tone, but they don't actually add anything to the story.
watney in the book is a funny character, that's why he manages to survive for a year without going crazy. it also makes his despair hit harder in the moments he's not wisecracking.