I would say don’t. My ex was into me as a person but I was a skinny flat chested sort and his ideal was fat and curvy. I didn’t want to be like that but it’s dreadful to be loved “despite your looks”. Really bad for self esteem.
It worked out in the end, not with him but the lean body type survives life, pregnancy, etc. without as much change (small don’t fall!) and when I got older, more guys were into it because I’ve aged well, and I feel better looking now - not in an absolute sense but relative to others my age.
Basically - you don’t find them attractive, be a good friend not a lover. Just because you don’t like their looks doesn’t mean nobody will. They might be someone else’s type.