The page ResourceLoader/Writing a MobileFrontend friendly ResourceLoader module says: "We don't load common scripts".
It appears that MediaWiki:Common.js doesn't run in MobileFrontend, but the user's own common.js does run. I noticed it when Echo started breaking on my mobile phone when I was logged-in, and the reason was that my User:Amire80/common.js used functions that aren't available in MobileFrontend.
I'm not sure why is the personal script loaded even though the site-wide common.js is not. This seems inconsistent. It may make sense to have personal (and site-wide) scripts run in mobile mode, but this should be happen in a more consistent, expected, and documented way.