There are varying degrees of easilyamusedness. (damn, now I have to update PamelaWords too...) Such things as SleepDep, HumPapers and FroshChem are often factors in determining one's level of easilyamusedness.
For example, the comments "Look, there's a pink elephant in the laundry room!" or "See you in class tomorrow!" would likely be relatively amusing no matter your mental state. However, it takes a certain mood (or personality) to roll on the floor with laughter at the terms "student body" or "light fixture," indicating a high level of easilyamusedness.