The best way to open a college essay depends on what you're writing about, but using humor is totally acceptable as long as it's thematically consistent. As you said, the point of the essay is to showcase your personality and what makes you special. If you feel that your sense of humor is a big part of who you are, and you can relate it to the main focus of your essay, a funny line can be a great hook.
The two most effective ways to begin your essay that we recommend in our article on how to write a college essay step-by-step are the "in media res" opening and the "specific generalization" opening. An "in media res" opening is a line that launches the reader right into the middle of your story, so it grabs their attention right away. If you're interested in the medical field, you might choose to write about an EMT program that you did in high school (not saying you have to have done this, just an example). Imagining this scenario, your "in media res" intro might be something like:
"Surrounded by a cacophony of sirens, I threw myself into the task at hand."
A "specific generalization" opening is when you make an overall statement linked to the thematic content of your essay that also includes interesting specific details that pull the reader in. Here's an example:
"My interest in medicine began at a very young age. As a toddler, I successfully extracted many a fluffy appendix from my unfortunate stuffed animals."
Hopefully these examples give you some ideas. I'd recommend reading the article that I linked to in the second paragraph of this answer for even more detailed advice!