The Hobbit is also a whimsical children's book, and doesn't have anything to do with saving the world (a world that Tolkien had not developed anywhere near the state in we see in LoTR when he wrote The Hobbit almost 20 years earlier).

Somewhat whimsical, yet somewhat grappling with dark undertones, possibly due to the trauma of the war.

The moomins starts with a great flood that washes them all away to live in a new place (I think this is a parallel to the Finns moving out of Karelia after the war. I believe this was the largest migration of people that had occured at the time, and it has been described as causing generational trauma to the Finnish).

In addition I believe MoominPappa deals with issues of depression or something?

There is an enormous difference in tone if you actually read any of Tove Janssons books. The animated moomin series is childish and cute. The world of the books is dark and scary and contains monsters and threats that are almost lovecraftian. The moomin trolls are victims to their surroundings and the forces of nature...

Are you sure you haven't confused these books?

One of the books you mention is about an adventure involving a treasure. The other book is about catastrophic flooding in the first book and a comet that threatens the planet in the second, if I recall correctly. Which one did you think was about saving the world and which one was about whimsical non-issues again?

Of course, you don't have to like the books. They are both children's books. But of all the possible critique this one was particularly strange.

Comet in moominland is about them learning about a comet heading towards earth that they believe is going to kill them all.

It wouldn’t be “Tolkien lovers” who are upset at these, it would be people too narcissistically self-involved with their own preconceptions.