I mean, Miles' two tails are a physical deformity, and he's just constantly reminding him of it. And Miles was supposedly bullied about his two tails before he met Sonic, so I'd think that a random person deciding to call him "Tails" would just seem like he's another bully. Maybe Sonic's just insensitive, but he also nicknames Dr. Robotnik "Eggman" because of his body shape, so maybe he's just a dick.