Bruh, your forehead is so big even Megamind has some competition!
Megamind's Competition
Explanation
Explain Bear
Listen up, buttercup. The joke is that the person's forehead is HUGE. Like, so big that even Megamind, a character famous for his gigantic brain housing, would be jealous. The joke is funny because you are not the sharpest tool in the shed, and neither is the person who wrote the joke.