This is mostly on David Stearns, and everyone in baseball knows it. But after Wednesday night, after the New York Mets endured a six-error embarrassment in a 10-5 loss to the Chicago Cubs, the need to change managers was obvious.The Mets finally and mercifully ended Carlos Mendoza’s tenure on Friday morning, a sad outcome for a skipper who only two years ago held such promise. Mendoza still might end up succeeding as a big-league manager, but it will be with another club. Mets Malaise, a condition thought to be endemic to the Wilpon ownership but has since extended into the Steve Cohen era, swallowed him whole.Mendoza certainly wasn’t blameless, and Wednesday night’s error-fest, in which all four Mets infielders put their own horrific spin on hitting for the cycle, was effectively the last straw. Yet even the capper to the team’s current six-game losing streak, as much as it screamed “fire the manager!” was as much a reflection on Stearns as it was on Mendoza.Stearns, the Mets’ president of baseball operations, ripped up his team last offseason, motivated more than anything — don’t laugh — by a desire to improve the team’s defense. The loss of free-agent first baseman Pete Alonso — a homegrown Met, popular success in New York and perfect right-handed complement to Juan Soto — might go down as the signature mistake of the Stearns Era. And it was only fitting that fans at Citi Field, amid the wreckage Wednesday night, took to chanting Alonso’s name.