Pipermail (example) needs a viewport tag for mobile browser (Android and iPhone) to use the responsive UI.
I think Mailman v3 interfaces do solve this issue.
In T190054#5636899, @MarcoAurelio wrote: I think Mailman v3 interfaces do solve this issue.
Indeed. hyperkitty is responsive / mobile friendly.
Except a very few mailing lists, all are now migrated to mailman3, I call this done.