>Therefore it makes sense to me that
there would be two adjectives in the next sentence: the
borogroves were mimsy, and the mome raths were outgrabe.

Well, Jackie, I see your point and there is a certain literary elegance in translating it that way.
But surely Lewis Carroll, meticulous in his punctuation, would have put a comma before outgrabe if it were an adjective? In addition, Humpty Dumpty's definitive transliteration of the poem certainly uses the word as a verb " ... something between a bellow and a whistle, with a sneeze thrown in ...," (or something like that - I quote from memory, not having a copy to hand at work.)
So I must to agree with Bridget.