Not sure what you have in mind, but MXML is based on the Flex SDK, hence Action Script. This can certainly not be edited as a design element in Lotus Notes.

The closest thing in Lotus Notes/Domino 8 are xPages. However, xPages are based on the currently supported Lotus Notes design elements. You can include include Java based components here (e.g. DOJO).

So on a Notes Designer level the answer must be NO.

If you had in mind to simply edit/modify MXML files using Lotus Notes as an XML editor you probably can, but it makes very little sense to me.

If your question related to how to launch a Flex application from a web-based Lotus Notes database, then the answer is to attach the .SWF (flash player) file created by Adobe Flex Builder to a document in the Notes database and add a link to it somewhere in your website.

The FlexViewer demo for instance is stored in the $about document of the web-service database required for the FlexViewer to read data from Domino databases. The $about document is set to launch the first attachment. So as soon you hit the web-service database it launches the FlexViewer (.SWF) for Domino, which in turn accesses the web-services in that same database to display the Notes data.