The mobile app doesn't make it easy to search for duplicates (compared to desktop site). Is it still advisable or nessesary to do so before asking a question? My thinking is only if it seems like a question that would be asked a lot.
It is advisable to search for duplicates, yes. Necessary? No, but you may create a duplicate if you forgo that step.
I agree, the mobile website and the app do not have a great UI for searching material, but in your mobile browser, you can load a mobile Stack Exchange page, scroll to the footer, and click 'full site'. I find that it's sometimes useful for navigation from my mobile, in instances like these.