For what it's worth, while Jim himself is voiced by a white actor (go-to Disney guy Cliff Edwards, who was also Jiminey Cricket and some other roles), the other three are accomplished black actors and musicians of the time. Including the very important and renowned
Hall Johnson. Another is James Baskett, who was a big stage actor, collaborator with Louis Armstrong, and in fact played Uncle Remus a few years later, which won him an oscar two years later in 1948 - the first ever awarded to a black performer.
I'm not saying that justifies anything, and it's unfortunate that Jim couldn't be voiced by a black actor/musician too. But it seems like there's more nuance here than meets the eye. I don't think the sequence is supposed to be mocking at all... the musical number sounds a lot like 40's/50's vintage blues, especially some of the more uptempo Delta Blues. If you listen to say, some Willie Dixon or some early Muddy Waters, it's a sound that was very much dominant in the genre:
Again, that's not to say the crows are a good look through a modern lens, or that they don't totally play into what Spike Lee called the "Magical Negro" trope: yeah, it's kinda nice that they're basically portrayed as the smartest characters in the movie, but that's kind of one of the issues too... they're a plot device whose cleverness is used to get the white (coded) Timothy and Dumbo out of trouble and teach them (and the audience) a
valuable lesson.
I dunno, all that is just my dumb take on it.