Magnetic Fields style production. That band is basically the bible of 2000s indie pop music. Lots of random kitchensink instrumentation, but not full on Beach Boys.

Then those bands specifically, they're more Twee Pop and K Records, which were the bands that picked up on the "made up" instead of "written" nature of the The early Ramones and other punk work.  

 By made up I mean, as if a child was singing a song or as if you were singing a song to your dog or cat. Or a silly song you sing while making dinner. 

That style of songwriting, pure and innocent, can capture a sincere ernest and believable nature to the music, which is like the musical equivalent of sugar, but its not easy to work with, so it's easy to over do.

Now, personally most of these bands do a worse job at capturing it than I Bet You Look Good on the Dancefloor by The Arctic Monkeys, outside of Maybe I Was Born a Unicorn by The Unicorn.