The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
A Shout-Out does not necessarily have to reference anything popular or even known. If you read the description, it says that it can be obscure.
It seems a bit too obscure for it to be a Shout-Out...Buffy was wearing what would be Kim's casual outfit for all of half a minute. Then again a scene in Witches where cheerleaders are set on fire is commonly said to be given a Shout-Out in one KP episode. I just found it funny, watching KP first and Buffy second.
Currently reading up My Rule Fu Is Stronger Than Yours
I was watching one Buffy episode and at the beginning she is dressed exactly the same as Kim Possible, specifically the lime tank top and blue pants she sometimes wears. I don't think this is a Shout Out from KP creator Steve Loter, as it would be fairly obscure. I'm leaning towards Hilarious In Hindsight, given the similarity of the two shows, but I'm wondering if there's a better trope for it.