I would not rely on the shows for canon when it comes to the mirror universe. in one episode two klingon ships decloak, but in a later one Quark has to steal one from the prime universe.
I still like my pet explanation that the Romulans are dealing cloaks while pitting people against eachother (ie. they were dealing to the Terrans until Spock took over, then they dealt to the Klingons, then they started arming the Terrans again) and the Romulans go through and wipe all traces of their involvement, which is why people "forgot" about cloaks. (Ie. the Romulans assassinated everyone who had a cloak and scuttled them all whenever they switched sides. Presumably under that model, the Klingons who had a cloak had them installed without informing KCA command.)