I think this disproportionate usage on desktop is just part and parcel for how #Steemit works best. Long form articles and in depth discussions are common place here. It is rather tough to tap out a long response or longer article on a cell phone and not have it riddled with typos. That is probably why more people use the site on desktops, where keyboards are common and a mouse makes it easy to navigate.
I bought a cheap tablet, Bluetooth mouse and keyboard for it (total about $65) so that I could work on articles and long responses while at work (on breaks and at lunch for instance). I simply got tired of trying to do more than give a few word replies using my cell phone - I am sure I am not alone here.
Maybe once the "mainstream" discovers Steemit we will begin to see more "selfie" articles and more "status updates" that are apparently still all the rage on Facebook and other social media platforms. I just hope that stuff doesn't drown out the actual intellectual stuff many members have produced now and will produce later.