The quote that this question is referring to appears in chapter 16 of Bud, Not Buddy. The chapter begins with Bud waking up after a long night of sleep. He's been needing a good rest. Bud quietly begins making his way downstairs, and he overhears all of the band members talking about him. Bud quietly retreats, and he then makes a bunch of noise upstairs. This alerts the band to stop talking about Bud before he comes down again.
When Bud arrives downstairs, Mr. Jimmy asks Bud what the "scoop" is. Bud doesn't know what he is talking about. Then another band member tells him to "cop a squat" and points at a chair. The pointing lets Bud know the man is talking about sitting. Then Miss Thomas asks Bud about whether or not his ears were "burning" last night. Bud is completely lost and lets readers know that he thinks people from Grand Rapids talk funny.
The band members aren't talking funny. They are simply using a lot of words and phrases that Bud has never been exposed to before. As the chapter continues, the band members start throwing in musical jargon like referring to a saxophone as an "ax" and "bread and butter." The band's musical world is completely new territory for Bud, and the vocabulary isn't something he's learned; therefore, Bud just thinks they talk funny because none of it makes sense to him.