Right now, I'm writing this post on my phone. It works fine, but it's still a bit clunky as all mobile sites are. Thus, Steem would really benefit from a mobile app.
I would work on a Steem app. I already made an Android app mysef (https://play.google.com/store/apps/details?id=getvague.com.gateway) and I will be willing to work on it as an open source project. Would anyone else have any interest in a mobile app?