Depends what you want to do.
Emails and Pages can be responsive if you use the correct Template, or set the Template to Mobile using the setting in there. It's better if you ask your designer to make it a responsive in the first place. See templates.marketo.com and docs.marketo.com for more.
Raul Fonseca wrote a nice article on how to identify/check leads that read content via mobile here: Build a Lead Performance Report with Mobile Platform Columns .
However, Josh Hill is right that you don't want to only show mobile responsive templates to someone who is on mobile. Your templates should be ready to provide the experience that the user is using at that moment!