It really depends on what your going for.
For monster helms, you have the basic idea, but your execution could use some work. It all seems very haphazard, and if thats what your going for who am i to argue, but you can do monster without haphazard.
From a basic construction standpoint i would start stitching your helms, or at least the main parts. Rivets are great for certain things, decoration, detail, hinges, but not for load bearing seams. Stitches make a helm look much smoother.
Your edges and cuts could use work as well. It looks like you drew a pattern free hand onto leather and then cut it out. It's ok, i do that sometimes, but more often, when i'm making a helm, i build the whole thing out of paper, then card board, and then finally when i have it perfect, i transfer it to leather and go from there.
When doing a two sided symmetrical design i draw i build it out of cardboard and paper...then i cut it down the center, and throw one half of it away. It doesn't matter how good at drawing i am, im not a perfect artist and i do make mistakes when drawing. By cutting a design down the center you can take the one half you save, flip it over, and make side two from the same pattern achieving symmetry
The one dome helm you have looks off...probably because its not a dome. An easy way to get a dome pattern is to take a ball cap that fits you well, cut it apart at the seams, and use the pattern from that to make yourself a pattern you can transfer to leather.
Hope that helps