Coming from a bit of a country girl, where farmer's tans are acceptable even for women, farmers tans can be attractive. It just depends. I see nothing wrong with them and find them awesome. But some people would have stupid comments and say they weren't attractive. But for me... lol.
They are unattractive if you wear a tank top when you have farmer's tan. But actually if a guy has farmers tan and I see it because he scratches his shoulder or something it sort of gives me a good impression. Like he spends a lot of time outdoors having fun or working and being all manly.
They are fine by me. I live in the country, where everyone has one, so we are quite used to them and not bothered by them in the slightest. It shows you were outside with a shirt on for a while. So what?