The term "Mickey Mouse" is associated with a beloved cartoon character, and gas masks are serious protective equipment. It's highly unlikely that such a name would be used for a life-saving device.
It's possible you've heard a nickname or slang term for a gas mask that sounds similar to "Mickey Mouse mask", but it's not a widely known or documented term.
If you have more context about where you heard this term, it might be easier to understand the origin.