Here is a quick test: the doodle was created in Moho 12.5 and contemporarily opened both in 12.5 and 13.
The shapes are clearly crisper and curves are much softer in 12.5, hands down. The feather-shaped brush I used for the beard is obviously better visualised by Moho 12.5, in Moho 13 the pattern is jagged. The overall display is worse in 13 (and these are vectors, for bitmaps it gets worse). Or could it be the trial version?
The doodle is drawn with vector brushes, the apps are open side-to-side, same zoom percentage. In both cases, GPU is turned on, preview modes are identical, and an empty 'neutral' layer was highlighted to prevent any difference in quality on the layers containing graphics.

P.S. I just tried a reversed test, creating a similar doodle in Moho 13, exporting it as Moho 12 and reopening in in both apps, side by side as in the previous example. Same scenario: graphics are perfect in Moho 12.5 but jaggy and angular in Moho 13.
This is particularly bad when one is working at full stage (zoomed out). Small objects will look clunky and without detail in Moho 13. For example: the features on a character's face will not have enough detail to be accurately animated.
PP.SS.: In a bout of doubt, I set a content folder (as Moho 13 prompts you to do on startup). I hadn't done this yet, being still in trial mode, but did so just in case that could've been the origin of the problem... but no, same thing.