If by ‘good’ you mean even remotely optimal, Quirky is a bad nature for Mismagius. Natures like this that are completely neutral and don’t boost any stats have almost no use in competitive play. Mismagius is no exception, as it’s almost universally optimal to use a nature that at least decreases its Attack and boosts a useful stat.
It’s not the absolute worst nature you could have gotten — Adamant or Brave would have been pretty miserable — and maybe you could get away with it. But if you’re concerned enough about getting good competitive Pokémon as to bother with IVs, then you should absolutely try your luck again for a good nature. If you already have a 6 IV parent, then you’re part of the way to getting exactly what you want.
Is your Misdreavus a female? If so, catch a compatible male with Timid, equip it with an Everstone and breed the two. You will eventually end up with a Timid male Misdreavus, which you can then pair with your first one and start focusing on IVs. Replace the male every time a Misdreavus hatches with better IVs, until you get one with six as well as Timid.
If you’re having trouble finding a Pokémon with the right nature, try using Synchronise. If your Misdreavus is a male, you can breed it with Ditto and get a female and then breed it with your first one, and do the same thing as above except with the sexes reversed.
