Personally, I couldn't care less what I am called by an FA. I do not find much need for formality in life and have no qualms with being referred to in any way it strikes the FA's fancy.

That said, I think it best for an FA to use Mr. or Mrs. (or Dr., etc.) when intorducing oneself to a passenger for the first time. If the passenger prefers something else, let the burden rest on the passenger to inform the FA.

I suppose you can never be chided for being too formal, but some people, for reasons I do not claim to understand, probably have a problem with too little formality.