I'm sure this also depends a lot on the field within mathematics. Areas like mathematical logic or algebra have a pretty formal style anyway, so it's comparatively less effort to translate proofs to lean. I would expect more people from these areas to use proof checkers than say from low-dimensional topology, which has a more "intuitive" style. Of course by "intuitive" I don't mean it's any less rigorous. But a picture proof of some homotopy equivalence is just much harder to translate to something lean can understand than some sequence of inequalities. To add a data point, I'm in geometry/topology and I've never seen or heard of anyone who uses this stuff (so far).