How to make categories collapsed by default in mobile view ?
Depends if you want to follow the easy way or the hard way.
The hard way implies dealing with source files, being the question in themes development I suppose the easy way fits better.
I think you can just "display:none" on the class .category_boards (provided this class is not used in other places, and I'm not sure it is not used, sorry but I don't know all the bits of the template, that's more for Spuds :P).