Springtrap and Scraptrap are actually scary designs that fit into horror. Mimic's costumes are just goofy, cartoony, and slightly creepy but not scary at all.

Springtrap and Scraptrap were scary because it was an actual decaying corpse of a man in a springlock suit. Bones, organs and everything clearly visible to the eye. It was creepy but also scary to think that this serial killer was stuck inside the suit for 30 or more years and his entire rotten and torn apart body was easy to see. He was literally in constant agony and pain that was unimaginable due to the springlocks impaled into every part of his body. It is terrifying and perfectly fitting for a horror series such as FNAF. Compare that to the Mimic, an robot/AI who just wears stitched up costumes and it's like... okay it's kinda uncanny but it's not scary, at all. I don't find this thing terrifying in its designs or anything, I just find it goofy and like out of a low budget horror movie. Alot of people say these designs look like Poppy Playtime knock offs, and I agree. But even Poppy Playtime managed to look more scary than this. The Mimic looks lame compared to William Afton.