With the present functionality of the mbox.js code, auto-loading the mbox.js code via the integrated Target tool could break scripts currently being deployed later in the as they may be pushed down into the section of the page.
If this issue is encountered and cannot be resolved by changing the load order of the affected scripts, there's always the option to revert to a Manual code configuration in the Target tool in DTM (when you do this, the hosted code will be retained and will become available in the 'Custom' editor in the tool settings).
Then, you can open the editor and move the createGlobalMbox() function out of the mbox.js code and place it in a page load rule to avoid sequencing issues with other scripts.