Now I realize Mono isn't the malware here. It's a legitimate platform that enables a certain degree cross platform functionality. You know what also allowed things to be cross platform? Java - and look how that turned out. Java is notorious for being a vector of attack.
Even without the never-ending stream of vulnerabilities that came with Java, the simple act of allowing executables from a highly targeted platform (Windows) run on one whose defensive ecosystem isn't nearly as well developed is just asking for trouble. Especially now that the cat is out of the bag.