Losing to good teams is just as bad as losing to bad teams, it’s just a different bad. The goal of baseball is to be the best, and anytime anyone asserts themselves as better than you it should hurt.
Losing to crappy teams is usually a comedy of errors, or running into the handful of good players the opposition has or the crappier players you’re still forced to play. It’s usually a sloppy affair. Getting so soundly slaughtered by the best teams in the league however is demoralizing because it makes it seem you’re so far away from being good. Even when you play pretty well you can lose pretty soundly.
The Mets, presumably, are trending upwards with young talent, playing .500ish baseball since the break, and looking to finish a place above where they did last year with maybe a better record as well. It’s hard to remember to that when you’re getting so summarily trounced by the Detroit Tigers or the Los Angeles Dodgers.