I think it really depends on demographic and subject matter above all else.
I check in here on Meta mostly on mobile, but move to my laptop whenever I need to make a lengthier response. I’ve been known to swap device and add to initially short cell-based responses from my desktop. Younger users OTOH are typically more inclined to use the cellphones by default.
Chat applications encourage shorter more frequent responses, Discourse includes specific tools and plugins to discourage such behavior if you deem them undesirable.