The term "overrated" is the go-to option for people who don't know enough about what they're talking about to be able to formulate an actual argument. Lots of stupid people claim that something is "overrated' if they don't like it, but heaps of other people do.
What is it that people say about Iron Maiden? Well, for starters, there are the facts - the band has always been comprised of incredibly talented musicians. The guitarists have always been top-notch, the bassist is widely considered to be one of the best bassists, if not THE best bassist in Heavy Metal history. Their music was way ahead of its time, they've been a major influence and a driving force in Heavy Metal for decades. They've managed to sell albums and pack arenas even as rock music has taken some very serious hits these past few years. There's a consistency to their sound that practically no other band could match, with the possible exception of AC/DC or Judas Priest...
Many of the band's supporters would say that they're one of the three most important Heavy Metal bands of all time, they're always on the lips of practically every important metal musician when he's asked about his influences. Musically, they've developed and progressed and evolved enormously since their inception, and the complexity and range to their catalogue is impressive.
What do naysayers have to say? That they're "overrated." As if the opinion of some dolt who can't think of a more valid or intelligent argument might actually matter. Listen to "To Tame a Land" or "Rime of the Ancient Mariner" and tell me that those guys are "overrated." They've had a few duds in their career- make 16 albums 35 years and that's bound to happen.
But they deserve each and every modicum of respect, admiration and prestige that they possess.
Simple answer; Just laugh in that person's face and walk away.