You're as useless as... insults. Not everyone's as useful as you so here's some ammunition to tell someone exactly how useless they are.
You're as useless as a piece of pork at a Jewish wedding.
You're useless as tits on a snowman, because even Frosty had a carrot nose that served a purpose.
You're as useless as a professional snail trainer with a speed obsession.
You're as useless as a master chef who can only cook ramen noodles.
You're as useless as a time-traveling historian who always arrives late.
You're as useless as a chocolate teapot in a caffeine-free zone.
You're as useless as a superhero with a fear of heights and a penchant for procrastination.
You're as useless as a chocolate teapot in a desert survival kit.
You're useless as tits on a robot, because robots can do actual things, unlike you.
You're useless as tits on a mermaid, because mermaids can swim, and you can barely tread water in life.
You're as useless as a knife without a blade.
You're as useless as the second shift button on the keyboard.
You're as useless as deodorant to cab drivers.
You're as useless as a remote without batteries.
You're as useless as a fart in a space suit.
You're as useless as a chocolate fireguard.
You're as useless as a keyboard without keys.
You're as useless as a underwear to Tarzan.
You're as useless as a phone without a signal.
You're as useless as pickup lines to George Clooney