I'm always of two minds about this kind of stuff.

First of, the illustrations are great. I love them.

Separate though, if they don't represent the original material then why not just make some new IP instead if effectively taking a piss on someone else's?

I don't think it's pissing on the source material, it's adapting it.

Alastair Reynolds once expressed this sentiment in a nice way:

    I didn’t want to be slavishly bound by the earlier story. So I made the decision that House of Suns would take its cue from the events and characters in the shorter piece, but it wouldn’t be afraid to contradict them if that made for a better story. 

The "original material" was modified significantly - Tove's illustration of Gollum, e.g., was not inconsistent with the 1937 edition she was working from, before Tolkien rewrote the scene to bring it more in-line with the version of the character from the Lord of the Rings in the second edition.

They do represent the original material, as interpreted by the illustrator. And Tove was hardly pissing on anything - she was commissioned to illustrate a version of the book by the publisher.