As a skinny person (and target of attempted mall abduction when I was 21), I don’t find it offensive. But I don’t find it funny, either. It sort of falls flat. I do think you’re over-reacting, but if you called the shop and pointed out that their sign might creep out some college parents (moms), they might see your point and come up with something better for their sign. (They also might think you’re crazy and over-reacting.)
I suppose you could also say it is “fat shaming”-- because aren’t they implying that fat people are not attractive to sexual predators?