It's not that bad... It's a good picture, it's just not entirely accurate, or it's drawn in a style that doesn't please my eyes...
I don't know specifically how to improve it as I haven't drawn her or Milotic much... And when I draw Seviper it usually turns out in such a way that I don't like it as well.
I find her face unappealing, which is partially because of the style you used, but also because it's so scrunched up near the top. Her mouth is kind of smirking, but her teeth are showing more like it was just an ordinary smile. Also the red on her cheeks/under her eyes or whatever doesn't help at all. There's either too much of it, or it's just placed badly. Though if you really want to, you could just say she's bad at putting on make up... Her armpit on Seviper's side is also a bit odd. I wouldn't have drawn that line there.(Actually, I might've, but I would have also regretted it) Not as long, at least...
Seviper's bottom jaw is too small. There's plenty of it. It's big and golden. The golden bit's on it's forehead don't look pointy enough to me, also, but I don't think that's necessarily what is making it look odd to me. But I'm not sure that I can specifically describe how to improve it. I would personally just look at an official picture or three again to get more of an idea how to improve. Of course, I don't much like the way Seviper looks when it's extended out even in the show, but I think what's really getting me here is just the bottom jaw.
And while I'm at it, the bits that drop down from Milotic's head are supposed to be longer. Not that big of an issue, though I think they are maybe a bit under double the length they are in your pic...
And in my defense, I was probably in a rush and didn't know exactly how to improve the picture so that I would like it. I wanted to comment and say that he was a good drawer(or at least colourer), but that I didn't like the picture itself. The number of people that comment on here rediculously positively is... a large number. I knew that he wouldn't have any trouble receiving positive comments and thought that rather than telling him "Perfect! 10/10!(message too short!!!!)" that I would tell him that I didn't like how it turned out. I hope I have explained myself better now, though I wouldn't be surprised if I didn't just get a few more grumpy responses...
You can colour nicely, undoubtedly, and you can probably draw well too(it's certainly not scribble), you could just use a bit more accuracy.(I find the same thing about some of the best pictures around here, though to a lesser extent) Keep up the good work. :)