After some searching, I found another solution to properly render only the wireframe of the triangles, here it is:....
func render(renderCommandEncoder: MTLRenderCommandEncoder){
renderCommandEncoder.setTriangleFillMode(.lines)
renderCommandEncoder.setRenderPipelineState(RenderPipelineStateLibrary.PipelineState(.Basic))
renderCommandEncoder.setVertexBuffer(mesh.vertexBuffer, offset: 0, index: 0)
renderCommandEncoder.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: mesh.vertexCount)
}