Apparently, the mobile editor lacks the ability to mark edits as minor.
Is that deliberate? Why, if I may ask?
Topic on Skin talk:Minerva Neue
It's just never been prioritized. I think there's also an element of how to fit this into the UI without being confusing - I don't think it's widely understood by the average user what is and what isn't a minor edit, but the phab ticket is pretty quiet so I think it's more likely a case of priority
(Also note this is an issue with MobileFrontend and its editor not Minerva - which has allows for minor edit editing)
https://www.mediawiki.org/wiki/Skin:Minerva_Neue?action=edit&useskin=minerva