The interface is very simple, I bet it works great on a phone through a browser. You need to be able to connect to someone's node though.
A web interface has already been developed (not by the core team, but by another NXT user who liked FreeMarket). They still need an SSL certificate, but here's the address:
https://freemarketlite.com
That might work well on a phone browser, I am looking forward to trying that out
Nice!
The site doesn't look bad on a phone browser. The list item form looks really good.