Lemon is mentally disabled because he is a lemon.

Lemon is also really stupid because the number 21 is a sign of stupidity as seen here

He also seemed to predict the ’21’ meme. The reason why he was screaming when he

predicted it is because he did not like that meme.

Lemon appears to suffer physical pain when his lemon offspring are harmed, as seen in the video MILEY CYRUS – WE CAN’T STOP (PINK GUY).

When he said “21” three times, it could possibly a reference to down syndrome. A person who has down syndrome has three copies of the Chromosome 21. Down syndrome involves mental disability, thus “I’m mentally disabled ’cause I’m a lemon”.