I'm a fucker and don't really mind being labeled as such. At least it's something that's honest and truthful in my case. Labeling someone as mentally ill is libel.
This time it is your bias that is showing.
Labeling someone a fucker is a meaningless, generic, profane insult with no content. Labeling someone as mentally ill is an insult with content and is considerably more specific. Whether it is libel or not depends on the truth of the matter. On that score, I would note that Eskimospy doesn't seem to ever label any other conservative poster here as mentally ill, other than Spidey. At a minimum, I'd say it's a perception, whether accurate or not, based on Spidey's behavior on this discussion board. If it was invented from whole cloth due to raw ideological bias, I'd expect him to apply the label to other conservatives. And he isn't the only one to have made this observation.
Let's put the shoe on the other foot. Notice, for example, how many people around here have suggested that McOwned isn't terribly bright, but those same people may or not say the same thing about other left wing posters? You think he has been chosen arbitrarily to receive this insult, or has his behavior perhaps led to it?
FWIW I don't think you're (usually) a fucker. There's about 10 people who post here that I have pretty negative opinions about apart from my disagreements with them, and about half of them are on the left.