In terms of artistic improvement, I think what would help most are the following...
1) A closer study of anatomy. This would help fix any issues with proportions, as well as make the characters look more natural in general. By "anatomy", what I mean is primarily examining muscles, and how they change with movement and different poses; it's a solid foundation for anyone who intends to draw people, regardless of style. It also helps avoid any unintentional "But people don't bend like that!" issues in the future, something that most artists have probably done at least once.
2) A closer study of perspective. It's particularly an issue with human subjects, since there's so many fiddly bits that an artist can get wrong, and people tend to notice even minor distortions in this area. Fortunately, there's a way to "cheat" in this area - make use of a 3D model to pose as a reference. It can be an actual doll (so long as it's realistically proportioned) or it can be a virtual model like those included in most higher-end artistic software (I think it's most? The one I used did, but I only ever used the one...). Either way, it will produce immediate results, while still helping you develop an intuitive sense of what areas are visible or distorted in a given position.
Both of which build in to what I think is most important, personally...
3) Shading. The characters all look rather flat at the moment, because none of the minor bits of shading that usually help define the shape of a naturally rounded and squishy human form are present at the moment - not even on Theresa's dress, which should have dozens of shadows. There are many different styles you could use to apply these shadows, to be clear, ranging from detailed gradients to simply applying a second tone to the most shadowy areas... But having some kind of shadow applied to the appropriate areas would probably have the biggest results on the game's art at the moment. If you have any doubts, I'd strongly recommend spending an hour or two just playing around with shadows on Theresa to see the effect.
Personally, I'd rather the time be spent writing more than drawing more - but if you're interested in improving the art, I think these would be the best areas to focus on.