No, it's not a fault. As long as it isn't excessive (such as extending over one or both eyes) white on the muzzle/head is acceptable.
As she gets older, most likely her nose will turn completely black and she will get black spots on her lips; this is the case with 95% of the white or flashy boxers I have seen. A handful of them keep a tiny bit of pink somewhere on the nose. I have seen only one picture of an adult white boxer whose nose was mostly pink. Caney's muzzle was all pink when she was born; then when we took her home at 7.5 weeks, she had a few small spots on her nose that looked like a paw-print

and now her nose is all black, and her lips are spotted.
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Zoë
Creek - Fawn Female, born 03/27/2001
Caney - White Female, born 03/26/2007
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