Dewey and Della singing a duet of "Stand Out" from A Goofy Movie and bonding is rather ironic; In the film, much of the plot was happened because Max performing that song got him in trouble and led Goofy to try to bond with his son, but their father-son relationship got worse after that. It's the complete opposite here.
At the same time, it's incredibly appropriate since the reason Max performed the song in the first place was to impress Roxanne (who already had feelings for him). Dewey does the exact same thing with Della in this episode, the only difference is he's looking for parental affection rather than romantic.
Also regarding "Stand Out": A Goofy Movie played in theatres in 1995. Presumably, in-universe, Powerline was a popular singer in The '90s, during Della's childhood / teenage years, and she just introduced her son to her favorite music.
When Scrooge's mother mentioned Dirty Dingus Mc Duck, Huey and Louie chuckled. Turns out Dewey's middle name is Dingus. He doesn't find it funny because he's probably had his own share of teasing.
