A neutral nature like Quirky does nothing for your Pokemon's stats. Even if you can't decide which stat to lower, the 10% boost from a proper nature is too big to leave on the table. You're leaving a free 10% stat boost on the table by keeping Quirky.
Quirky is a neutral nature with no stat change. For special walls, use Calm or Careful instead.
Calm
+Sp. Defense/-Attack
Maximum special bulk for special-move walls
Careful
+Sp. Defense/-Sp. Attack
Maximum special bulk for physical-move walls
No Pokemon foundNo Calm or Careful nature Pokemon are available in Winds & Waves.
Quirky is a neutral nature — no stat is increased or decreased.
Competitive Role
No competitive advantage. A stat-boosting nature is almost always preferred.
How to Get
SynchronizeLead with a Quirky Pokemon that has the Synchronize ability. Wild Pokemon will match its nature. (Gen 3+)
BreedingGive the parent with Quirky nature an Everstone. The offspring will inherit it. (100% since Gen 5)
Nature MintUse a Quirky Mint to change stat effects to Quirky. The original nature name stays but stats change. (Gen 8+)
Recommended Alternatives
What does the Quirky nature do in Pokemon?
Quirky is a neutral nature. It doesn't raise or lower any stat, so your Pokemon's stats stay at their base values. In competitive terms, it's the same as having no nature effect at all.
Is Quirky nature good?
How do I get a Quirky nature Pokemon?
Three ways: (1) Breed with an Everstone on a Quirky parent. (2) Lead with a Synchronize Pokemon that has Quirky. (3) Just catch anything. There's a 1/25 chance it already has Quirky. But seriously, you probably want to change it.
When should you use a neutral nature?
Almost never. The only edge case is a Pokemon that uses physical moves, special moves, and needs every defensive stat and Speed. That Pokemon doesn't really exist in competitive. Even mixed attackers prefer Naive, Hasty, or another nature that at least boosts Speed.