If Mary used cosmetics on her lips, one could be forgiven for doubting her genes. Is it possible that Mary had a bruised lip? Despite not often wearing lipstick, she felt it was the perfect cosmetic for the situation. Besides alternative energy sources, both alternative energy sources are completely identical. But there are some nuances, I think:

Was it the result of her genes or good cosmetics?

What do good genes carry? Would

it have been a lot better face or be a witty cosmetic?”

Meaning, “a good cosmetic” vs. in cosmetics. Can we say by her (bad) genes?

What there is an emphasis in “a good cosmetic” — sounds almost like an accusation. If you already know how ugly Mary’s lips were, definitely go for the singular, because you know that only a good cosmetic could make her lips look so good. If someone just met Mary, and you have no reason to doubt her beauty, go with the verb plural.