Memorable film scores are usually thought of as being big sweeping affairs; the orchestra swells as the Death Star does its death-y thing, or the strings go nutty as the murderer closes in, or the flutes get all ethereal when the lovers look at each other.
Music. It's dramatic.
In You Can't Take It With You there's some background oomph in a couple of places and said oomph created by Dimitri Tiomkin, a Russian conductor and composer who worked on many of Capra's most famous films. But the most striking music Tiomkin makes here isn't big set pieces. Instead, it's diegetic.
What is diegetic music, you ask? Is it some weird 1930's genre that sounds like goats stuffed to their goat gills with extra-strength laxatives? No, not at all. Diegetic music is just music that is part of the action on screen. When Grandpa and Mr. Kirby play their harmonica duet at the end there, all the other characters can hear them, because the music is part of the story. That's diegetic music. (No goats in sight.)
All the best musical moments in the film are diegetic, like when Ed plays the xylophone and Essie pirouettes around, or the harmonica duet between Kirby and Grandpa, or the scene where Tony and Alice dance with the street performers (until the police interrupt them). The best musical scenes in the film are the characters enjoying music.
And not great, polished, sweeping music either; rather, they enjoy music that is small-scale, plunky, folksy fun. The film loves creativity, but not high-falutin', pompous creativity. Instead, it embraces creativity that all the folks can enjoy. You Can't Take It With You loves family entertainment, so it's no surprise that the best music from the score gives the family on screen something to dance to.