Sorry for being a bearer of bad news, but the "subcategory" view is broken on mobile devices. (At least on the lavender theme.) Users cannot tap to open the different categories, and sometimes the categories overlap each other.
I've tried this on mobile Chrome, mobile Firefox...even mobile Opera and the result is the same. If someone could point me in the right direction I'll start work on it and make a pull request.
Note: If you want to simulate this find a nodebb subcategory page with many subcategories and resize your browser window to approximately the size of an iphone's screen. Try clicking the subcategories and you will see the issue.