What does the Quirky nature do?

Quirky is a neutral nature. It does not raise or lower any stat.

Can I change a Pokémon to Quirky?

Yes. Use a Quirky Mint to apply the Quirky stat effect to any Pokémon without changing its displayed nature.

Nature

Neutral: no stat is raised or lowered.

No stat is raised or lowered.

Filler nature; no stat trade-off.

What does the Quirky nature do?: Quirky is a neutral nature. It does not raise or lower any stat.
Can I change a Pokémon to Quirky?: Yes. Use a Quirky Mint to apply the Quirky stat effect to any Pokémon without changing its displayed nature.