I would agree that the A6M had a feared mystique that the allied command had to destroy. The legend of the 'zero" was a priority of the allied airforce...
As much time and effort that went into destroying the legend of the A6M...was also put into building the Spitfire up.
Very good points; one of the keys to 'overrated' or underrated is the efforts of war time propaganda and media accounts, the flavor of which still influenced aviation writing in the West (and everywhere else, but let's focus on the West for now) for decades after the war, even now affects attitudes. Most people here have mainly read or started out reading at least in English and other Western sources.
As you imply, during the war itself it was actually important to the Allied war effort that the Zero be talked down, not viewed as 10 foot tall opponent, lest it permanently damage morale, of pilots or general public. The narrative that was selected was one where Japanese fighters as epitomized by the Zero, and as a symbol of the the Japanese military in general, got in a few quick and unfair sucker punches against unprepared Allies, but then the 'good guys' quickly turned things around. The Zero quickly turned to a pumpkin: a story with limited truth to it. As has been discussed on this forum, there are numerous examples of Japanese Navy and even Army fighter units spanking Allied ones even in 1943, later still in some cases. The standard story where the Zero fades somewhere between Midway and end of 1942 into an easy to defeat opponent is a mix of literal interpretation of honest but quite serious overclaims, and just plain warmed-over war time propaganda. It accelerates the truth by 1-2 years. And it ignores how the Allies brought their enormous superiority in resources to bear more effectively as the war went one (the resources to build a multiple as many planes, the resources to carefully train a multiple as many pilots); plus matchups of the Zero increasingly against Allied a/c which entered service considerably later. And those were the main factors bringing down the Zero, not 'tactics'.
The Zero is probably the most underrated major fighter of WWII, certainly among a few candidates for that spot, from Western POV. It might reasonably be called overrated in Japan where accounts based on their own overclaims are sometimes still published as fact, but it's simply an ill informed choice to put it up for most overrated, from a Western POV, to put it bluntly.
Mirror image for the Spit, it was important for the British and Allies to believe strongly in that plane. Its shortcomings, insofar as they were they seen through the fog of overclaims, tended to be glossed over, and that still filters through in British writing especially, and a lot of English language writing about British airplanes is by Brits. The 'Spitfire' is really a series of pretty different airplanes. The early ones were pretty advanced compared to contemporaries (and saw important air combat), so were the later ones (but they didn't see a lot of really important air combat). But the Mid Mark Spits were pretty mediocre compared to most of their opposition, and almost surely had a true kill ratio v German fighters well under 1:1 (even early Spits probably below 1:1 too, but not as much and were accomplishing other important things). The mid Mark Sptis also had a disastrous record v the Zero, but that's a fairly minor footnote. The Spit has to be at or near the top of the list for most overrated WWII fighter from Western POV.
In both cases a lot of the reason derives from the after effects of Allied war time propaganda on English speaking readers.
Joe