Not just humor, but the overall tone of your application essay is remarkably important. It's also difficult to get right. When you are asked to write about your accomplishments, those 750 words on how great you are can make you sound like a braggart. Be careful to balance your pride in your achievements with humility and generosity towards others. You also want to avoid sounding like a whiner -- use your essay to show off your skills, not to explain the injustices that lead to your low math score or failure to graduate #1 in your class.
After I had eaten the small portion which sufficed to fill my stomach halfway, Brother David casually mentioned his belief that it was an offense against God to leave food uneaten on the table. This was particularly the case when such a great restaurant had so clearly been placed in our path as a special grace. David was a slim man and a monk, so I found it hardly credible that he followed this precept generally. But he continued to eat so much that I felt good manners, if not actual spiritual guidance, required me to imitate his example. I filled my belly for the first time in a year.