I found this review of the Samyang 300mm f/6.3 mirror lens being sharper in the corners than in the middle. Is that a common characteristic of mirror lenses?
Mirror lenses, as well as catadioptric and Newtonian reflector telescopes, all have an obstruction in the center. This leads to a loss of contrast in the center of the field. Contrast is intimately related to acutance, which many folks call "sharpness."
Even traditional refractive lenses with field curvature can be focused for the edges to be at their sharpest, rather than focused for the center to be at its sharpest. If the field curvature is strong enough, this can give sharper results on the edges than the center of a flat test chart when so focused.
The reduced performance in the center of the field could also be due to using a spherical mirror (cheaper and easier to make) instead of parabolic mirror (more difficult/expensive to make) and focusing optimized for the edges instead of the center when the test images were made.