Invader Zim has always been a good show and the drawings have always been unique. I think for your fan art you've created a good use of lines and it is nice to see you used the robot elements here rather then just a simple "ZIM" drawing of him standing or whatever. I think there could be a background here, maybe of his house...and where is "GIR" we need to have him in here, there is no Zim without his Gir. Love the coloring, love the shading, love the positioning and all. Im not the biggest fan of this style, but I always keep the bias out of my reviews. But I must say this did catch my attention to some point so nice job on that part.
~~THINGS TO IMPROVE ON~~
You do need a background and Gir should be in there somewhere.