I want to say yes, and even on this very board. I believe it was…
@burrunjor ? that posted a discussion about this topic beforehand some time ago.
Thank you, I'm sure it was, though I'm also sure I'm not the only person to make that comparison.
I'm always interested in looking at how works inspire other works.
Joss was a huge X-Men fan and has said that Kitty Pryde was an inspiration on Buffy, and also said that Buffy's surname Summers was named after Scott Summers, whilst Dark Willow yes was by his own admission inspired by and named after Dark Phoenix.
To be fair though other just a team member going evil, and the name and motif of a bookish red head turning into a dark haired villain the stories aren't all that similar. I think it was more just the basic template they used in this case.
Incidentally I think that Dark Willow is actually a very influential story in that there seems to be a trend after her where a young, nerdy, bookish, or awkward female member of a team is taken over by some dark power, gets white skin, dark eyes, and goes completely insane becoming the main villain for a while. Prior to going mental she often has a period where she thinks she can use the power for good only for it to take her over.
Examples include, Chloe and Brainiac from Smallville.
Killer Frost from the Flash
Even Angel got in on it with Fred/Illyria.
The actress will normally list that story as their favourite for obvious reasons LOL.