You know what the surest sign of an MMO’s popularity is? Whether or not World of Warcraft fans are defensive about its popularity. That apparently came up on Reddit when someone ran a search with Google search trends and found that, under specific criteria, World of Warcraft is much more frequently searched than any other online game! That post did all right on the WoW subreddit, and then it went over to the main MMORPG subreddit, proving that WoW is the biggest game out there.
The actual answer, as put forth in the second post, is that it proves absolutely nothing. Search trends are, well, search trends, and so many of them rely on precise wording of what one searches for; all of these particular searches include variants on the game names, but the author of the second post even points out that if you look for searches for “WoW” you get a whole lot of background noise for searches that have absolutely nothing to do with Warcraft or worlds thereof.
The problem is compounded when other acronyms overlap; for example, both The Elder Scrolls Online and Black Desert Online have acronyms that tend to overlap with other search terms, thus making them nigh-on impossible to gauge through trend analysis. Really, all you can find by comparing how many people search for “FFXIV” to how many people search for “World of Warcraft” is… a comparison of two different numbers. It’s not indicative of anything beyond which terms are searched for more often and not a predictor of overall popularity.
On the plus side, learning about all of this prepares you for a future in analyzing trending topics, so you can derive some information from these goings-on. Just not about the most popular MMO out there.