Jacket of All Trades
A person who tries to be good at everything, but ends up being bad at everything.
'She's a jacket of all trades. She tried to be a singer, a teacher, and a baker. Now she just bakes bad cookies and sings in the shower.'
'He's a jacket of all trades. He tried to be a lawyer, a mechanic, and a magician. Now he's just confused and always late.'
'My brother is a jacket of all trades. He tried to be a dancer, a chef, and a rapper. Now he just dances badly, burns food, and raps about it.'