If you code your page to be responsive you do not need to enable Marketo's mobile functionality. That is really just a workaround for people who don't know how to code proper responsive Marketo landing pages. It also causes you to need to duplicate all the work you do in the editor.
We find most of the time if people enable Marketo's mobile functionality it will actually break your existing landing page.
Hope that helps. Feel free to follow and DM me for more help.