I’ve always had an awful experience with mesh importing and exporting and here’s why:
first of all, if i ever make a mistake and don’t have any blend/mesh copy i can’t edit back onto blender cause the edjes break or have some sorta of permanent shadow flat look
here’s my character but i brushed the smooth tool a little bit:
Meshes are imported into the MeshPart at whatever angle the meshpart sits.
I normally get meshes that are 180 degrees out, but they still are centered on the same axis so the Rotate tool can be used to make them the right direction.
Roblox recently announced (New Unified, Smart Mesh Importer Beta) they are releasing a better tool to improve mesh importing/exporting with changes to the size and rotation of Meshes as well as other new stuff. You probably would have found the post with the Search tool up top.
you can rotate your mesh in blender and then it will be imported with that rotation applied (so just do in blender R>180>Enter)
To fix those shadow problems go Material Properties > Scroll down to “Settings” > Blend mode to OPAQUE