Hi there rise, just thought i would give you my two cents.
When most people start pixel art they are not very good at it, aside from a few talented lucky individuals who start off good and take off quick... for most of us its practice experiment practice experiment all the time. Forums like this help of course as many of the members will be able to pass down their knowledge and experience.
So stick around if you plan to get good at this tricky tricky stuff!
onto your art!!!
I notice that your tile size for the blue tiles is 10 x 10 pixels, that seems like an odd size to me. and then your square with legs takes up 15 x 15 tile. Now normally in games tile sizes are either 8x8 16x16 24x24 or 32x32 or some kind of variation along the same line. i beleive this has something to do with processors being able to handle these numbers better due to bits and bytes and whatnot... something along those lines if i vaguely remember, of course these days it matters less... (if anyone knows the exact reason feel free to refresh my mind)
Anyway, your purple tiles i dont have a clue what size is going on there!... I suggest you change your tiles all too 16 x 16, not only will this allow better details in larger tiles. it also should lead to easier and better programing if everything is in the same tile size!... i think.
So assuming you do change your tile size to 16x16 as i sugest, you might not for whatever reason. there are several ways too improve your tiles!
I have made an edit so you can see, while coffees edit shows you a much better 3d version. i am going down the whole "classic square look" while still making it modern.
With 3d graphics in controll of the game industry, 2d graphics have the challenge of conveying 3d. i find in the best cases 2d graphics depict 3d with better charm and style than actual 3d renders.
you can see by making all the tiles 16x16 everything is unified in size.
I have rid black outlines as they are not a good idea.
Think about the materials your tiles are made of... some random blue substance? i decided to go for metal panels and added a bit of shading.
I gave the heart an outline because everything else had one.
I conveyed your character to be 3d as well as the heart and tiles for some depth.
I used a solid shade on each cube edge because the surface is flat not round.
I shaded accordingly too a lightsource, something impossible to do too non "3d" objects...
you might find some other details in the edit but i hope this has helped you improve these i am looking forward to seeing an update. try to be adventurous with the tiles make new ones you might not want metal panals, add variations to make it interesting. explore and experiment and see what you get!

there are of course still lots of problems with my edit the tiles are too distracting and other problems but it should give you a general hint!