Websites built with X5 will automatically be displayed at their optimum due to the code produced by the software, irrespective of whether the end user is browsing on a desktop or handheld device.  There are some templates amongst the presets which are specifically designated for 'mobile' use, although it's not strictly necessary to use them...  all the default templates are optimized for a 1024 x 768 pixel display, whether described as 'mobile' templates or otherwise.

However, you can if you so wish create a second version of your site, hosted on an additional domain, or a subdomain of your first site.  You could then use various methods to detect the display resolution of the device being employed by the user to view the webpages, directing those on a low-resolution screen to the second (mobile) version of the site as appropriate.  This is a huge subject in itself.  Do a Google search on 'responsive design' for detailed information.

Whether it's worth the effort, or is an essential thing to do, remains your personal choice.  Some feel that a mobile version of a site should be customised to have less text, larger icons, swifter navigation, etc.