Most Helpful Guy
The one I'm crushing right now is plain looking to many, below average for some. So yes, some guys are attracted to plain looking girls.
I think it's like an instagram filter in our mind, we tend to make them look much more beautiful in our head. XD
Most Helpful Girl
Some of them can be attracted to "plain-looking" girls. However, I don't think that's the correct term for it. For example a guy may call a girl "plain" while another may find her very attractive.