Why pick on Effi Barry for spending District money to attend the Cannes Film Festival under the guise of the ''First Lady'' of the District of Columbia {"The Cannes Job," editorial, May 16}? The Post hasn't bothered to gripe about the excesses of the first first lady.

Remember when the Reagans went to Moscow? First Lady Nancy had her chief of staff and entourage dispatched to Russia no less than three times to arrange her schedule. Wouldn't one (or no) time have been sufficient? (Anybody remember what Mrs. Reagan did in Moscow? Of course not.) How about when she made a trip to Scandinavia while her husband went someplace else? Wasn't her entire itinerary vetted before she arrived? This was about as big a waste of tax dollars as paying for a full-time hairdresser for the millionaire first lady.

Who picks up the tab for Marilyn Quayle to fly hither and yon to learn about ''disasters''? What is she going to do with this knowledge if she gets it? We have a professional cadre of men and women to conduct the nation's business. We do not need a bunch of wives of elected officials trying to do the job of these professionals.

As Hatty Babbitt pointed out in the presidential primaries of 1988, the term ''first lady'' is an insult to all the women in this country and should be discontinued. Instead it gets worse, if you consider that we now have ''first daughter,'' ''first son,'' ''first granddaughter'' and on and on ad nauseam.

The Reagans, Bushes and Quayles are multimillionaires or very close to it. The money spent on these people could go a long way to ease the burden of someone who needs assistance.

If it sounds as though I resent royalty, you've gotten my point. This country fought in 1775 to do away with bestowed titles, such as first lady. And any reference to any titles is thoroughly antidemocratic and wholly offensive to the taxpayers who underwrite this expensive folly. JACKIE ENDRES Silver Spring