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Definitely get her cleaned up like Finchly said to avoid any bacterial infections.
Gently hold a damp cloth there and then gently wipe. Don´t pick and tug at feathers as it may hurt.
If you bird enjoys bathing in a bowl, she may be able to clean herself.
I would take your bird to a vet because a messy bum could be a sign of an infection. Clean as much off as you can, and hopefully you can see a good vet.