Western society's conception of adulthood is steeped in toxic masculinity, careerism, neoliberal individualism, and social conformity. "Man up." "Be normal." "Don't be colorful." "You're too old to play pretend." "Buy a house." "You'll understand [my conservative bullshit] when you're older."
What we actually understood as we got older was that the older generation had no business telling us what was and wasn't mature. We've seen these Good, Respectable, Normal People support bombing villagers on the other side of the planet. We've seen them support racial profiling, indefinite detentions, and torture facilities. We've seen them crash the economy chasing the idea that everyone must own a house. We've seen the wretched job market we graduated into after being told that graduating college would secure us a good career. We've seen companies squeeze us dry and then lay us off after we'd been told "work hard and you'll get rewarded." We've seen them destroy the biosphere because they couldn't tell the difference between weather and climate. We've seen them throw a collective fit over wearing masks to inhibit the spread of a deadly pandemic. We've seen them endlessly parrot "if you have nothing to hide, you have nothing to fear" as every aspect of our lives grows increasingly surveilled by a state that's hostile to our existence. We've seen neurodiverse and LGBT friends who only found true happiness when they embraced the parts of themselves that others demanded they outgrow.
Given all that, unashamedly enjoying and displaying cute aesthetics can feel a lot like a middle finger to the people who demanded we grow into the most drab, joyless existence imaginable and turned out to be full of shit about everything else.
And no, before anyone tries to claim this is what I'm saying, this doesn't mean I think that buying Marvel Funko Pops is anti-capitalist praxis. 
"Capital has the ability to subsume all critiques into itself."
Japan's "Kawaii" culture developed in a similar way.