It looks like there is a decent resolution, and your tablet settings are probably fine if those are the default. Visually it looks like you
are painting with MRGB on instead of RGB. This causes your strokes to also paint material data into the polypaint, and unlike normal vertex colors painted materials don’t blend as nicely in real time (you basically get to see one material per polygon, vs polypaint colors blending between vertices).
If that’s the case and you don’t need several matcaps on the model, then switch your brush’s paint mode to m (material), select the Flatcolor material and use color: fill object to erase the material information while preserving any polypaint RGB color. You can then select another material of your choice to be seen on the mesh, and go back to painting (be sure to switch back from m to RGB).
If you want to have several different materials on the same mesh, then you can either try increasing the blend radius to affect renders only, or else try splitting the surface into polygroups with some loops around the borders, or split each material into its own subtool
