|It completely protects its wearer from the effects of clouds.|
Note that some cloud-generating breath weapons also inflict impact damage when they hit you (some of which may also come with status conditions); a scarf of cloud immunity does not protect you from this damage. Examples of this include catoblepae's Petrifying Cloud, swamp dragons' Poisonous Cloud, and ushabtiu's Death Rattle.
Cloud immunity is an underwhelming ego except in the Swamp, where it allows you to avoid the breaths of swamp drakes, swamp dragons, ghost crabs and clouds associated with some Swamp:4 rune vaults as well. Generally it will act as a good substitute for poison resistance here. Having a scarf of cloud immunity can also come in handy in the Volcano, the Ice Cave, the occasional vault that generates hazardous clouds, or if you plan on creating clouds in battle.
Otherwise, even a regular +0 cloak will often do more than a scarf of cloud immunity by providing 1 AC.