Mariah Carey is a more legit rapper than rapboat.
Mariah Carey Rapper
Explanation
Explain Bear
Listen up, buttercup. This joke says Mariah Carey, that diva who hits notes higher than your GPA, is a better rapper than someone called "rapboat." It's funny 'cause Mariah's known for singing, not rapping. You probably don't even know who "rapboat" is, which kinda proves the joke's point, doesn't it?