I think providing a source when adding a new common name should be required or at least strongly encouraged. That would reduce issues with people adding a new name that doesn’t make sense and nobody’s ever heard of, or names that would apply equally well to 20 other similar unnamed species (e.g. “aster leafminer”).
However, I’d also encourage people working with obscure groups to create checklists and identification guides and create common names to make that group more accessible to people. Then those names can gain traction and be added to iNat by other users.