you really need to decide with the developer. For what it’s worth, for most of my mobile projects (which is 75% of my work at least) I deliver mono audio SFX files. There’s really no need for the sfx (UI/weapons/hits/rewards etc) to be in stereo. Usually the music loops are in stereo, and I mix those on apple buds since a lot of people will be listening on those. I try to be sure that I am the one who delivers not only uncompressed audio, but also the compressed or optimized files so there isn’t some producer using garageband trying to do that on their end. It’s not so much about the device and whether you can replicate stereo on it’s speakers, but more about the game mix environment. And if the dev is using unity, those mono assets will be in a 3d space anyhow, so you wouldn’t want them in stereo.
There are also the different ways that android/iOS process audio files and the streams that are available (in addition to file types…android likes ogg and iOS doesn’t for instance.)
hope that helps, but be sure to always talk it over with the developer/team