Hotjar comes with built-in support for responsive designs that change automatically as the browser increases or decreases in size. This is achievable since Hotjar tracks visitor usage relative to the elements on the page, not relative to the page itself. This means that if elements on the page shift position on different resolutions, Hotjar will still report the correct usage of those elements.
Your Heatmap report will include a Heatmap for three different devices - Desktops, Tablets and Phones. This is particularly useful if your site design changes based on device.
At the top of your Heatmaps the device selector allows you to view the Heatmap for each device:
If you do NOT use responsive CSS and instead have a separate mobile version of your site which still matches the targeting set for your Heatmap, the mobile version of your Heatmap should still load the correct site layout. If the separate mobile version of your site is accessible via an alternate URL, like m.example.com or mobile.example.com, and you are using targeting which wouldn't match with these alternate URLs, data collection for mobile devices on the Heatmap will be affected.