Why is it there is always an evil genius in the Rogues Gallery of heroes? And why do they have to be so annoying?? To be fair though, very few are as smart and as annoying as Gizmo.
Gizmo is like that little brother who won't shut up, except when he's not annoying you, he's making gadgets that can likely kill you. His childish personality aside, Gizmo really is smart. He's made robotic legs which can help him climb up walls, he has a jetpack, holographic projectors, bombs, and more. He's given the Teen Titans a run for their money multiple times and has personally gotten the best of Cyborg a few times.
So yeah, he's annoying, but at least he can back it up.