JACK-OF-ALL-TRADES
A person who knows a little bit about everything, but not enough to be called good at anything. They’re like a half-baked cake, still edible, but not great.
He can write, design, and sing, but he can't be called good at any of them.
She can paint, cook, and dance, but she can't be called a pro at any of them.
He can fix cars, build decks, and mow lawns, but he can't do any of them well enough to be proud.