This shouldn't really be surprising, I'd think most people's internet usage is probably on their phone, and has been for some time.
People don't want to sit at a desk or whatever and browse or do their socials stuff, they want to do it sat on the sofa while the TV is on or in between chores in the house.
I think the last two companies I've worked for, both B2C have had mobile web and app usage way higher than desktop web.
Such projects desperately need some professional UX/UI people involved. They always feel like they're designed by back-end devs.
I also can't imagine there being any difference to the instances as the concept of "local" doesn't really make sense like it does with Lemmy. Maybe they should obfuscate the instance selection and the sign up process just puts you onto a random instance or one that's best suited by geographical location.