>>17499341In Black and White, they're hated for being the Wesley Crusher of Pokemon. Nobody would mind if they were just random Pokemon, but Black and White pushed them on you harder than the actual starters.
In X/Y, they're hated because the location they appear in just happens to be the location where you find out about Wonder Trading. It's no so bad now since you mostly get breeding leftovers, but when the game came out there were thousands of poor unwanted monkeys being shipped around the world.
Which is a shame, because they're useful in-game. Their movepools are pretty wide - Simisage learns usable Grass, Fighting, Dark, Rock, Flying, Ghost, Steel, Poison and Ground type moves - they don't care whether you use physical or special moves, and they make good users of all your pinch berries.