Is there anything known about the Dev improving the artstyle at some point?
I'm not sure what timeframe you are referencing, but there indeed have been 2 to 3 crucial points.
One was between 0.14.5 and 0.15. when DC basically decided to be take the game more serious and to actually care a bit more for the way he drew stuff (better shading, fewer orange colours, more quality in general), which then lead to 0.16.0 when he updated some characters and started to add the new, proper 12(?) frame animations, instead of the old 2-4 frame ones. The actual change happend with 0.17.0, when he started to introduce the rig-system to keep the game playable on mobiles. Everything that happened after 0.17.0 is supposed to be more or less on one quality level.
So in short, eventhing that came after 0.17.0 was/is supposed to be final and won't be changed, everything that came before 0.15. was/is supposed to be reworked/redrawn and everything in between (teachers and Roxxy) still needs a touchup.