I think some writers in the first half of S1 didn't care much or didn't have high expectations about its potential or weren't aiming for anything higher than a dumb comedy. And some writers/crew members saw more to it: Fortune Cookies, Mewnipendence Day, St. O's, and Storm the Castle's story outlines all had the same guy involved and I don't think that's coincidence. Two executives who helped develop it for television were also involved with the first season until halfway through, then left-- which is right when the plot episodes happen to begin. Some people got it and some people didn't.
Though for 2 I think the more filler episodes were done intentionally, to break up the lore/development/HABBENING episodes. Too much plot in succession doesn't let things really sink in; sometimes you just wanna see some characters' everyday lives so you can appreciate it more when things change. Doesn't mean all of them were good, but the balance between important and non-important ones was handled better this time.
It's probably because I really like where the show's gone now, but somehow imagining Star and Marco just fighting Ludo in the yard over and over for years feels like the saddest thing ever. This show really benefited from the lore.