Definitely not. But it depends on your definition of 'average looking' or 'unattractive.' What I think is average or unattractive, is probably very different then someone elses definition. I think we all have some type of attractive quality.
It's flattering regardless of what the guy looks like. Unless of course they're looking for a different reason. Perhaps there's a piece of salad in my teeth, or maybe they're secretly plotting my death. But if they're looking just to admire a girl. I don't see how that would be offensive in the least. Girls who wig out over this are nuts. They spend hundreds of dollars on make-up and hair accessories. And when a guy does look at them they automatically assume he's creepy, or mentally ill if he doesn't look like some type of underwear model. It's ridiculous.
I would respond by looking back, smiling, and giving him a little wave. But looks mean absolutely nothing to me. Maybe I'm not fit to answer this question.
looking is brief... staring is a different kettle of fish. And considering the context and what the girl is doing... is he looking because he gets a glimpse of cleavage or something similar or is he looking at her overall and or just her face.
Is his eyes wide open simultaneously tongue hanging out in a really inappropriate manner or is he keeping his cool and 'just innocently looking at a pretty picture'. Does he admire her and know or know of her on a daily or weekly basis or is she a complete stranger is another question.
Depending on these answers and maybe more depends on whether it would creep her out or be somewhat to some extent flattered.