Your mum's hairline was so big that Dora the Explorer could not find it.
Dora Can't Find It
Explanation
Explain Bear
Alright, listen up, buttercup. This joke is saying yo mama's forehead is so massive, even Dora, who finds everything, can't locate her hairline! Basically, she's got a fivehead, not a forehead. And you, well, you're probably still using training wheels on your brain.