Before I get to the critique, there are some technical issues to cover.
The first is that the sphere isn't perfectly aligned with the axis. This makes it difficult to work with, as anyone who uses it will have to adjust for the fact it is looking slightly off to the side. You want your values for rotation to be even values of 0, 90, 180, or 270.
The other issues is the texture. Did you really make the original texture yourself? Because the one you have here appears to be a .jpg that was scaled upwards and saved as a .png file, and I can't imagine why that would happen if you made the original in the first place. If you did in fact make the texture, I suggest making it at the size you want instead of making it too small and resizing it.
That's it as far as technical issues go. The OBJ and MTL are both fine.
The model itself doesn't look bad, but it's not very impressive, either. Anybody can put a picture on a sphere or a cube. When you plan on modeling an object that actually is just a sphere or a cube, such as this enemy here, you have to put more effort into it make it stand out. I'm glad that you manually fixed the UVs on the model, because it shows you're willing to put effort into it, and that's a good sign.
The texture isn't bad, but it could definitely use some work. The main problem is that it just isn't symmetrical. None of the curves on one side match up with the curves on the other side, making it look lumpy and misshapen. There are also a number of sharp turns in the curves, which you want to smooth out. You want the curves on both sides to be nice and smooth, and match each other as best you can. I also suggest using round edges on the mouth line instead of square, as it looks unnatural. I also suggest checking a reference. A quick look at TSR shows that the enemy's lips are, in fact, white. Other than that and the technical flaws, it's nice.
There are several ways you could improve this, and I'll leave it up to you on which paths you want to take. One of of the biggest problems is that the model just looks too flat. It looks good from the front, but as you turn it, you see that it is just a ball with a painted-on mouth. You want to somehow make the mouth stand out, so that it doesn't just look like a picture of a mouth. The best way, but also the way that requires the most effort, is to model out the mouth, so that it has nice polygonal lips sticking out of the ball's surface. You'll have your texture to help guide you so that it matches your original design. If you are unsure about making original models, there are tutorials you can find online that show you good techniques for making custom shapes.
If you don't think you can make polygonal lips (though I hope you try), your other option is to put lots of extra detail into the texturing. A bump map will help make the lips stand out, making the light hit them as if they were sticking out of the surface. Another thing you could do is add shading. Soft shadows below the mouth and along the undersides of the lips will make the mouth more convincing, as flat lips can't cast shadows. Combining this with a good bump map will make a nice effect. Or, if you really wanted it to look nice, you could use the shadows in combination with polygonal lips for a really nice effect.
There are a number of other things you can also do to the model to make it even better. A Specular Map would let you make the body more shiny than the lips, and prevent light from reflecting off the black crease in its mouth. Making an Environment Probe image (also known as a mirror ball image) will let you specify exactly the what the reflections and shiny spots on the ball look like when viewed from any angle. This would let you imitate the exact lighting style you see on the enemy's sprite, such as the double highlights and ring of reflected light around the edge. Another thing you could do is include an texture (or textures) for the recolored version of the enemy. If you don't know how to do these things, don't worry, there are plenty of tutorials out there.
Again, these are all options, and you don't have to do all of them, but I do hope you will at least try all of them. If you're really having trouble, I can help, but be sure to look for tutorials first before coming to me. I wish you luck in this, and I look forward to seeing you improve.