TextMesh Pro typically uses signed distance fields to cheaply represent fonts well at multiple distances.
Distance fields work best with smooth features due to their typically low resolution.
Increase the Atlas Resolution or Sampling Point Size until the quality of the text has reached a threshold you are happy with.
Do not increase the Atlas Resolution beyond 2048✕2048 when targeting mobile devices.
When manually increasing Sampling Point Size large values can cause characters to leave the atlas.
Using a different Render Mode (that isn't Smooth or Raster) may also produce better results in specific circumstances. SDFAA is the default mode.
Unity, huh, how?