I’ve requested this in 2018, apparently it’s currently being done, it apparently is taking some time though.
It’s really an issue, because almost all phones have multiple cameras now and as you’ve said, Mapillary seems to take the default camera that’s presented by the API, which is the worst one apparently. On my Xiaomi Mi9 it’s neither taking the wide angle nor the 40MP lense, instead it takes the default 10MP lenses which is a massive reduction in image quality compared to my previous P20 Pro. I’ve now bought a broken used P20 Pro again for a few bucks just to take photos, not only for Mapillary but also generally. Mapillary still doesn’t access the best possible image quality but it’s much better than the Mi9 at least.