How reasonable? I think pretty reasonable; MoR directly criticizes canon on numerous occasions, from the exchange rate to Hermione being Sorted into Gryffindor to Harry using random curses on Slytherins and on and on. Reading through one link on that, I see nothing about the Seuss parody parodizing Seuss, and plenty that fits MoR, eg.:

Parody achieves its status as social commentary by disparaging the original work, however slightly, by "pointing out faults, revealing hidden affectations, emphasizing weaknesses, and diminishing strengths.^1^

or

The court concluded that the infringing work broadly mimicked Dr. Seuss' characteristic style, but it did not ridicule that style. 170 The court noted that Penguin's use of the Cat's stove-pipe hat, Dr. Juice as a narrator, and a title similar to the original's title were all means of drawing attention to the new work, perhaps "to avoid the drudgery in working up something fresh."171

Finally, with regard to the purpose and character of use, the Ninth Circuit considered whether The Cat NOT in the Hat! merely superseded the Dr. Seuss originals or whether it "transformed" those works. 172 The court did not recognize any effort to create a transformative work. 173 As a result, under the first factor, the court concluded the scale tipped against fair use because the infringing work was neither a parody nor transformative. I74

...When considering a parodist's claim to fair use, a court must first determine if an infringer's work meets the threshold requirement for the defense: "whether a parodic character may reasonably be perceived. >7232 Courts have recognized parody as a work containing a discernible direct comment on the original. 233 Although the Ninth Circuit conceded Penguin's work did broadly mimic Dr. Seuss' style, it concluded that the work was not a parody because The Cat NOT in the Hat! did not target the "substance" of the original work.234