It depends. If she is just mildly unattractive then I'd still be flattered. If she's genuinely hideous (by which I mean like lower 5-10% of the female population), then it would be kind of disheartening simply for the fact that it's often the case that women only tend to hit on guys they feel are at their own level of attraction.
I would be flattered but obviously a more attractive girl hitting on you would get you more excited. Having unattractive people hit on you and be attracted to you is better than having nobody be attracted to you.
It is always an ego boost to have someone check me out. Especially because it doesn't happen very often for me :P most flirting I do is at work where we hit on each other like crazy to keep alert and energetic through long shifts. But if the unattractive person approached I would feel uncomfortable and wouldn't want to hurt their feelings. I'm kinda awkward about approaching and being approached but I'm getting better at it, I haven't dated much so my experience is lacking
It really doesn't matter if the guy who's checking me out is sexy as hell or looks like his face has been beaten in with a hockey stick...I CAN'T STAND guys checking me out. It's one of THE MOST awkward feelings for me. People have also bashed me on another comment when I said I wear pjs virtually all the time...why? Because they're comfy, easy to put on, and why else? Because I know guys won't be looking at me like they want to bang me! I know how to dress, but I'd much rather have a guy want to be my friend than have the intention of them getting in my pants when we start to talk. When I'm friends with a person we get to know each other instead of basing our relationship on something sexual or something that could lead to dating. Sorry about getting so angry and in detail about this, but people p*ss me off. lol.
if it's a guy that is a lot older then me (old enough to be my parent) I pull a disgusted face or if I'm in a absolutely horrible mood I will say ew if it's a guy closer to my age I either ignore the guy or give him a look that say f*** off.