Two things you can check…
In blender select all your mesh faces in edit mode.
on the left hand side there is the tools panel underneath shading/uvs there should be a tab which says … Recalculate normals. Check that tab. Do this before you export it.
Secondly
When you exported this mesh did you tick the correct check boxes down in the left hand corner. There’s a section which has a tab called (main) its highlighted blue. With your mesh selected in object mode check the box underneath that says selected objects. Then move your cursor down to the section which says … (Forward) make the selection for this (X forward) then choose z for up for the box below that one.
Next to (main) is a tab called (geometry) set smoothing to faces. Then export.