Are you getting confused between individuals and species here, or something? It's not harmful to the species for mate selection to produce better offspring. Peacocks exist, for example.

The tail isn't harmful to the peacock that grows it. It's helpful, attracting sexual interest. This is the paradigm example of sexual selection.

You're trying to defend the idea of evolving baldness so as to repel women, which is the opposite of the example you cite in "defense".

The more likely explanation is something like, higher temperatures impair cognitive function and older men were more likely to be in the village conducting business around the fire or in the presence of indoor heating than freezing their butts off in the wilderness as the younger men did, so they evolved to start having less insulation on their heads after they leave adolescence.

Or that some random mutations do not have evolutionary consequences. Also lot of powerful people have had lot of offspring despite baldness.

"Nature did this for no reason" is in general not the default assumption. It's not that it can never happen, but rather that as soon as there are two phenotypes, evolution is going to start selecting for one of them over the other in any given environment unless the difference has zero effect on evolutionary fitness, which is unusual.

My uncle had noticeable baldness very early.

When he graduated college and started working in finance, he found that the baldness was a major advantage, making him look older and therefore letting him avoid the stereotype that young people are immature / unreliable / ignorant / untrustworthy.

Not true. Traits that "harm" the individual can increase population level fitness.

https://pubmed.ncbi.nlm.nih.gov/29602452/

For a group selection argument, you'd need to explain what maintains strong reproductive barriers between the groups. That approach is doomed to failure for baldness, which has already spread to every major group and therefore couldn't be explained by between-group differences anyway.