in Just- spring when the world is mud- luscious the little lame balloonman whistles far and wee (1-5)
At this moment, the balloonman could easily share in the innocence of the scene. "Little" and "lame" are adjectives which may not be particularly desirable, but they’re sure not scary.
the queer old balloonman whistles (11-12)
OK, here’s where things start to get a little darker. The "queer" (we’re guessing that that just means strange here), "old" balloonman? Now he’s no longer part of the youth and regularity of spring.
the goat-footed balloonMan whistles (18-20)
We’re back again to the Pan references. Notice that this time, "Man" is capitalized. Why? Well, one thought might be that Cummings is trying to distinguish between the balloonMan and the children even more now than he was before.