I think you are off to a great start, I actually really liked the really simple small environments you did.
I do have a few pointers though that I think will quite easily and quickly take you much further than where you are sitting at now.
so first of I made this quick paint over here
I tried to paint out all the edges and vertices I see that are “fillers” you said you wanted to do mobile games, and in mobile we don’t want to waste our resources at all, we wanna be as efficient as possible with everything we do.
so when I say fillers(which by no means is a technical term) I mean verts or edges that is not helping describe any form or help with deformation. they are mostly just there taking up space when instead of just having random edges around the model we could use that detail to make the silhoutte less jagged or even add more details around the model, or just simply cut it down depending on what you need.
the axe is pretty good in this regard though, so good job! but the eyes are a very very high poly for what they need to be. Next up is the texturing, now this one is a bit more tricky as you might end up working for games where they use this very simple shading here, but generally, you would probably be well off with having just a little bit more detail or gradients throughout the model.
Next up the barkeeper.
actually the furniture in this piece is great man, really nice, it is exactly this kind of texturing I think you should be using more of to the describe what we are looking at. the problem with this piece is that the barkeeper is supposed to be the center of attention here, but I just cannot stop looking at the furniture, cause it is more detailed in contrast to the character. I went and checked your wireframe for the guy, and I can see he suffers from the same problems as the rest of your pieces with a lot of extra unnecessary geometry. but if you can take his textures to the level of the furniture you would be well on your way for a great mobile game character texturing piece.
the last piece you show here is just a continuation of the other problems I see, very flat texturing, and a lot of extra geometry. But I really like how you contextualized it with the image you made, it adds a lot to it.
for reference I made a quick picture with some pieces which does it right with a bit more shading and using every edge for either silhoutte or deformation. take a look
you are dooing good just keep pushing! there are a bunch of tutorials and pictures around the web where people show their method for making that kind of hand painted look- https://www.youtube.com/results?search_query=hand+painted+textures
all the best