It depends on extent, and whether it is noticeable in the final image. It can be due to magnified lens defects/little other contribution of teleconverter (TC), or it can be
that the TC is really poor in quality or is not suited to the particular lens. I use a Σ50-500, sometimes with Σ 2XTC, with no noticeable effect due to the TC.
Tim, I hilighted part of your post. The rest of the explanation is OK, but it would be a more helpful reply to expand on that statement as well.
Am.