Hmmmm alright
dis looks Coolio here is my tips :)
1. try practicing anatomy (specificly the skull) when you comfortable with realism try adding more stylized effects I onse heard someone say that realism is the rules of art and stylization is how we break them to be unique but it always important to have a base knowledge of the rules to better grasp the rules your breaking or bending in stylization
2. maybe try adding a little more shading i think it will ad some great depth to your art and it doesnt have to be anythibg crazy! A simple line of gray on one of the sides of the faces will do some wonders !
3. you used a thick brush for your line art which isn't bad! It's good for a simple cartoony style like the adventure Time of the amazing world of gumball :) but when doing this style it's important to have straight confident lines to do this I recommend the "lazy brush" it adds a small lag to your stoke making it eaiser to draw nice smooth lines !

Hope this helps !! X3

@zoni_lilly K! TYSM! I will try these now :D

yayyyy ! glad i could help :3 !