It is not something to ignore, the emerging market is not computers, it's phones. More people are using their smartphone to do things now. And XenForo is not embracing the times, but rather focusing on the old web model.
My last poll went somewhere from 5% to 25% for mobile usage, http://xenforo.com/community/threads/share-your-mobile-devices-usage.22040/ I expect that to go up. I also think this is a chicken and egg problem and people don't load the forum in a smartphone because it's so bad.
The first thing to focus on is the content, the rule of thumb is, if the page cannot be read once it opens, it is not mobile device ready. If I have to zoom to read content, it failed
This is a screenshot from my iPod touch. Look at the size of the title bar, look at the size of the location bar. Now look at the size of the text. Remember that the iPhone is 3.5 inches. I guess you *could* read it. If you really try. Hardly. Straining your eyes. And just one line.
Not everybody uses a mobile device. A lot of people however do. XenForo needs an edge in this. In it's current incarnation, xf is not mobile-ready.