I do not consider it a swear word. It is however, a coarse word as currently used, which I consider bad manners. I would not include it in the same category as 'swearing' in the religious sense, which would be more closely related to 'blasphemy' and considered by some to be a sin.

John Crapper was the person who widely distributed the modern toilet and his name became associated with it. So much so that the terms "go to the John" and "take a crap" became common. And vulgar just means common...

It's vulgar!