Not a mobile app but a mobile view and might be relevant:
https://community.zammad.org/t/become-a-beta-tester-for-zammad-6-0/11395/4
I don’t think a dedicated mobile app on platforms will come any point soon. Not that I can tell if it’s planned or not. I just don’t see the use and sense personally unless people pay for the cost (as not just development but also keeping it up to date etc costs a lot of money).