Yeah during the mage exam she demonstrated her childishness by... Taking control of the group, and teaching and protecting the two actual children that were on her team.
Or when she ran into the child Himmel in the forest, the writer showed what a child she was by... Maturely helping him find his way home and keeping him in good spirit with a magic trick.
But she does like to sleep in! That's undeniable! And no adult in history has ever liked sleeping in.
And Fern acts as a mother to her in the mornings, and when she's shopping / scavenging. It's a bit mutual, at this point.
(Though, yeah, I'm fairly certain that Frieren's childish behaviour is almost entirely intentional on her part, and that she can turn it off and act like an adult when she wants to, which admittedly isn't very often; she might also be a bit feral from all the time she lived alone in the woods.)
Nah. Frieren's appearance and behaviour are straight from the manga, which isn't that kind of book (seriously, the author just wanted to write about an unassuming elf John Wicking demons and accidentally turned it into a great story about death, and friendship, and whatnot) and only ever uses raunchiness in humorous contexts (the clothes dissolving potion, Flamme's “secret seduction technique”, Fern constantly considering everyone a pervert, and so on).
This isn't Miss Kobayashi's Dragon Maid (though that also accidentally turned out deeper than what it was supposed to be, which in that case was smut).
What Madhouse clearly are into in any case is feet (seriously, this show looks like a Tarantino film at times). Serie's, especially. And Übel's armpits, for some reason. Both of those are much more present in the anime than the books (also, excellent animation, like the fights or the dance scene, which in the books are usually just a couple panels; that's probably not a fetish, though).
I'm sure you think it's the same thing with the furries.
"It's not the ones who dress up in animal costumes for going to orgies and make porn of it that are into bestiality, it's the peiple who think furries are freaky that are into bestiality."