If your forum isn't mobile responsive, you're going to lose out. This is a huge SEO factor for any forum (and any website, for that matter). It not only affects SEO rank, but it’s also something that the Google Search Quality Raters look for when they assess the “Needs Met” rating.
If you have a AnswerCart forum, you're in luck.
AnswerCart is mobile ready right out of the box. A AnswerCart Forum is able to recognize when someone is trying to gain access through a smartphone. When this is detected, it automatically switches to the AnswerCart Mobile theme so that users on mobile devices and tablets can see your forum in a view that’s compatible with their device.