That sounds bug-like, if Cloudmin is calling it with --std-vga even though you don’t want that. I’d recommend filing a ticket in the tracker, with the exact command line Cloudmin is using to start your guest.
That said, we’ve generally focused on server uses of Cloudmin, so it isn’t too surprising that it wouldn’t handled graphical stuff well, yet. But, we welcome feedback on that and there’s no reason it shouldn’t be possible to run desktops in Cloudmin guests.