Maybe I can clear this up a bit. Different artists worked on the game for different periods of time. It's why the art-style often shifts dramatically from planet to planet. Alot of the early stuff was done by Gats and Cheshire, but now that Shou has been brought on as the official artist, it's slowly becoming more standardized, which is great.
We generally kick out new NPCs faster than an artist can keep up, so having Shou go around and replace the Tarkus/Tavros busts is unlikely, or at least long-term. If you'd like higher quality busts for a specific NPC, I suggest talking to her or Cheshire about a private commission.
Even moreso when she's had to do work with seasonal characters as of late, that are only present for a certain amount of time and won't be met until next year.