Templaters do not have access to less/css right now and that's why they use a class like .mobile It's a replacement for their inability to use media queries to target mobile devices and to create responsive templates.
MediaWiki:Common.css has stopped being sufficient for their needs, because it is shipped to every pageview, and thus only the most common usages of templating usually make use of MediaWiki:Common.css
Sadly T483 has stalled. It's a change that would require support of Parsing team as well as frontends standard group as from what I remember the RFC requested support of a <style> tag in wikitext.
@TheDJ What we would like to have here is actually not necessarily make template authors to come up with their media queries on their own in every new template, but exposing a simple set of queries, that we also support in core to them, correct?
Slightly related to T93675: Should be a standard way to access tablet browser width threshold