If one goes to the uncategorised templates special page:
One can see that those contain quite a few template styles pages.
One way to distinguish those is by the fact that they have a Sanitized CSS content model, as opposed to wikitext that regular ns10 pages have.
Uncategorised templates special page is routinely used to, well, look for uncategorised templates, for which being categorised is a guideline or a policy is a number of Wikimedia projects introduced to facilitate their discoverability. Template styles pages are not templates, and neither it makes sense to categorise them, nor there is a transparent way to do that, though putting a [[Category:]] code inside a /* */ CSS comment does work. Some wikis might prefer to do that, but many might want to just opt those pages out of uncategorised report.
Ideally there should be a configuration on what content models to include to or exclude from the report, ideally this should also extend to other uncategorised reports.
As a good enough option either Sanitized CSS pages specifically, or all non-wikitext pages should be excluded from the uncategorized template special page specifically.
Of course, it is now possible to obtain a list of uncategorised templates without the TemplateStyles pages by running a custom SQL query, but who would benefit from this feature would be users who do template maintenance and do not have necessary SQL knowledge or access.