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.
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.