But in terms of natural selection, certain characteristics are asserted to be BETTER (more adaptive) than others.
In natural selection, traits can be advantageous, disadvantageous, or neutral. Eye color, for instance, is probably neutral. A trait that in neutral in one environment can become disadvantageous or advantageous in another.
For example, black people in sunny Africa have an advantage because the melanin (black pigment) in their skin absorbs the UV light and protects them against DNA damage. But black people in the US don't receive much sunlight, especially in the more northern parts of the country. Most of the sunlight they do receive is absorbed by the melanin. As a result, it is difficult for their bodies to make enough vitamin D. Their bodies then produce extra cholesterol, so that more of it is available to make vitamin D with the limited sunlight they receive. But too much cholesterol is bad for the heart.
Conversely, white people do fine in the US with light skin; with a little sun, they make all the vitamin D they need because they have so little melanin. But white people in Africa are very prone to skin cancer.
By no means does the evolutionary hypothesis of natural selection -- that is mate selection preferring "better" characteristics - SEEK diversity.
If certain characteristics are more advantageous for survival, diversity is *BAD* because diversity means that some specimens have disadvantageous characteristics. If mating specimens are choosing a mate based upon the selection of "better" characteritics, to select for diversity is to promote WORSE characteristics.
There is absolutely nothing in the hypothesis of evolution or any of its dicussion that supports or even mentions mate-selection to SEEK diversity.
If you are seeking a mate, do you want your mate to be a close relative, or would you rather not be related to your mate? If you are married and your spouse isn't your sibling or first cousin, you've selected for diversity. If you marry someone from a different racial group, as a number of people do, then you have sought maximum diversity.
On a closely related issue, people also select for a mate most likely to deliver reproductive success (in other words, the most "fit" mate, in the sense of "survival of the fittest"). The traits most often associated with attractiveness are also traits associated with health and fertility. People aren't quite so analytical about their mate choices, of course. But the process of evolution has hard wired their brains to seek certain qualities in a mate.