Mark is too easily distracted and amused by little things
It reminds me, quite a bit about the types of comments that used to appear on my report cards in public school. Mark is a good student, but is too easily distracted by fellow classmates. Or: Mark is a bright student but often spends too much time trying to amuse his friends rather than pay attention to the focus of the class.
I suppose that's why, when I saw this video clip of Homer Simpson I thought: Poor Francine. she has to put up not only with the fact that this is the type of goofy thing I regularly do, but I'm also easily amused by it, and will spend hours of my time laughing and giggling when referring to it.