Why can't you be rude or sarcastic in your thesis statement?
Writing's a lot like life in general: First impressions count. Would you walk up to someone you haven't met and say, "Let's play horse. I'll be the front end, and you be yourself"?
To build a relationship — whether it be with a person you're getting to know or with the readers of a paper you're writing — you must convince them that you're sincere, informed, and interested in a respectful association.
We see examples of rudeness in our everyday lives — at school; at work; on the streets; at the movies; sometimes, even in our own homes. When we call an act "rude," we're saying that it was crude, lacking education, discourteous, unpleasant, or just plain mean-spirited. Who would want any of those words applied to a written work?
Similarly, sarcasm can be a complete turn off. Keep in mind that a thesis statement is designed to make a specific and accurate assertion about a topic. If your statement comes across as flimsy in its importance, weak in its approach, or nasty in its attitude, you'll have to work extra hard to convince your readers that you have a valid point.
Here's an example of a solid thesis statement: Female students learn better in all-women colleges.
Now, here's another take on the topic: Female students don't hesitate to speak their minds in all-women colleges . . . because they have nothing to lose.
The second example might make your readers chuckle for a second, but they'd probably doubt your ability to make a convincing argument. Your teacher likely would fail to see any humor (or value) in your "creative exercise."