The first Moomin book came out in 1945 and the last in 1993.
The Moomin animated series came out in 1990.
So it’s not unreasonable to believe, that Risotto Nero, leader of the hitman team, both read and watched the Moomins, which led to Metallica’s (Stand) Hattifattener design.
If I were to write a head-canon, that the same Risotto, who murders people for money, was reading the Moomins to at night in his childhood and probably owned a stuffed Moomintroll, it can chronologically make sense.
Now since he murdered his cousin’s murderer in 1991 (1973+18), it isn’t unreasonable to believe, that afterwards he went home, washed himself off of the blood, changed and sat down in front of the television to watch an episode of the animated series. Then cried about his childhood and loss of it, yet also finding a certain type of outlet for his pain in remembering it.
Metallica (stand) is in his blood to fill up the emptiness inside him and, as his childhood was killed when he was 14, it also fills up his empty childhood by looking like something from his childhood.
But, as he himself has become, it is twisted and grief stricken, stitched together after an unfortunate accident and never properly healed.