Yes, you can set it in a theme.
However you probably don't want to do it at an application level as it may prevent your users from being able to use pages that haven't been designed for mobile use as it prevents pinch to zoom.
The mobile controls plugin includes a mobile.theme file that does provide this tag for you and we also have a mechanism to enable it just for your mobile pages:
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Open the Package Explorer view in designer (it's most easily found in the Java perspective) and in your NSF open WebContent/WEB-INF/xsp.properties
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Add "xsp.theme.mobile.pagePrefix=mobile" to the end
This will override the Application's theme for any XPage whose name begins with "mobile" (eg mobileMain.xsp or mobileDocs.xsp) and use a device specific theme instead.