When you enable Nanite on a mesh, it also creates a fallback mesh. This means that it uses traditional LODs for the fallback mesh and made it too low poly. To adjust the resolution of your fallback mesh; under your Nanite settings for the mesh, switch the Fallback Target from Auto (default) to “Relative Error” and then set the Relative Error value to a low number (0.1 for example).
A lower number will create a higher-resolution fallback mesh. You can see the triangles of your fallback mesh under the mesh details shown in the top left corner of Persona.
I need to point out that THIS IS NOT THE ACTUAL FIX - If your mesh looks low poly with Nanite enabled, it means that it’s not using nanite for the mesh, but instead rendering your fallback mesh. So although switching to Relative Error and setting the value to 0.1 will seemingly fix the problem, it’s just an illusion because all you did was increase the polycount of the mesh that is being displayed (which you may want to do anyway mind you).
The actual solution is to figure out why your mesh isn’t using Nanite even if Nanite is enabled. The most likely reason is that your mesh is probably using a translucent material which is not yet supported on Nanite meshes. The solution here is to either separate the translucent parts of the mesh and make them non-nanite meshes, or to disable translucency on any materials that are applied to the nanite mesh. The fastest way is to disable translucency on your material (to make sure this is the problem) by opening your material instance, scrolling to the Material Override section, and switching Blend Mode from Translucent to Opaque.
Or if you’re not using a material instance, make this change in the master material.
If removing the translucent material doesn’t fix the issue, here’s a link to the currently supported features of nanite: