I overheard a "WTF is she dating that guy..." conversation directed toward me. What should I make of that?

Its either: - your looks (the shallowness of some people really shocks me sometimes)
- your behavior (you are known for not treating people right)

I suspect its looks since she is deemed at a really attractive girl, but you really shouldn't care what people say. She finds you attractive, so that's all that should matter. Not what bitchy, shallow people think. Love is love.
If its your behavior in the past, then simply try and prove people wrong by treating her like an angel, don't let anyone say something that could put doubt in her or your mind.
Next time, maybe go up to them and ask what the problem is? In situations like this, I never avoid confronting rude people in an honest manner, when people complained about my ex behind my back, I asked them alone why they thought that and their opinions turned out to be right. But you shouldn't let those kind of people to affect your relationship, gossip angers me :')

Congrats, you're punching above your weight.

People like to gossip and be snide. Ignore it. Who cares? As long as the two of you are happy in your relationship that's all that matters.

Alright, don't worry so much about what other people think. You probably shouldn't hold your "standards," you should fall for someone based on them being themselves around you not, let's say a list of things that you want to see. Anyways, next time, talk to your girl about it calmly. If you hear them again, confront them about it. Tell them that what they are doing is wrong and that they shouldn't be judging couples: ESPECIALLY out of appearance, if that is what you mean. Don't be ugly towards them, just simply state that is isn't right because what matters is between the two people that are together.

What should I make of that?
That it's possible that your girlfriend is more attractive to you (likely the guys) and/or your girlfriend could get a partner who treats her better (likely the gals).

So what should I do?
Probably do what most guys would do in your scenario and aim to do- keep dating the most attractive gal they can get.

" like I'm spitting in Darwin's face, taking the "fittest" women from the "fittest" guys. "
Egh perhaps you have a skewed interpretation of Darwin as he likely he wasn't since you're a fit guy you get a fit gal rather he was the fittest survive as in those who go out and get what they want.

"cause I'm tempted to lower my standards"
LMFAO I highly doubt that

Just pat your crotch and wink.

There was this girl I used to date, she was a stunning tall brunette that ALL men wanted, and when we were together I'd often hear people saying (sometimes not even caring I can hear them), "What is a girl like that doing with that guy" or "she could do so much better". It really got to me. I talked to her about it, I was really insecure. Eventually, it was MY INSECURITY that messed us up, and not what anyone else was saying. Whatever you do, don't lose confidence, don't show her you think you're not good enough, behave as if it's perfectly normal (and why wouldn't it be) for you to date a girl like her. Girls don't like insecurity. She's with you, she likes you, enjoy it.