Another way to view it is a series that accompanies a child to young adult years. (It took me a few years to get through the series inbetween other books. I assume many bedtime reads for the first few, and independent reading for later ones). What starts as fantastical and whimsical, indeed becomes more somber and concerning interpersonal dynamics, especially those within Moomin family and those around them. "Moominpapa at Sea" is probably most relatable to adult readers. The underlying message of family unity, finding ways to cope with certain events, parenting styles, need to break routines (picnics), unrequited crushes.

Moominland Midwinter is interesting character study too, with sporty "artistic" coded Hemulen proving too much for most of the cast to handle, the forgetful grandpa. Perhaps the message was it is okay to not be friends with everyone as it is a bother.

Compare it to "Finn Family Moomintroll" which is just a constantly stacking ridiculous lighthearted escapade with a few more mature jokes mixed in.