Something that I keep trying to do is right-click on a mockup in the left hand panel and then “export” it to PNG. Of course, then I remember this isn’t an option and I use the menu.
It would be really nice if I could do this though! Also if I could choose “Export to PDF” so just that one mockup goes to PDF. (And if you take my other request for “multi-select” in the mockups panel to heart, then I could use it to export just the selected ones to individual PNG or PDF)