I am not the original author of the plugin, I did the simple JS template it was based on and I'm helping out with some of the WebGL coding to make it more effective in mobile, etc. At the minimum, I think an ACE needs to be added for On Animation Finished.
For me, it has fairly reasonable performance on mobile on an iPhone XR and a Moto G6 now with the latest version of the plugin.
Check if you are using the latest addon from the direct github link: github.com/gritsenko/c3_spine_plugin/tree/master/dist
v 1.3.0 is the latest as of 10/10/2019, try it out on mobile and let us know how it goes.
For the skin issue, try leaving the skeleton name parameter blank. There is some difference between a JSON w/ multiple skeletons and a JSON with just one. In your case is the export of just one skeleton? I am usually over in the Construct Community Discord, if you want you could upload/DM me the *.json and *.atlas and I could try. This plugin is compatible w/ the 3.8 version of Spine.