As a fellow Supernatural fan, I think trying cosplay based on it would be a fantastic idea. The only unfortunate part is that the CW is more known for its teen dramas, placing Supernatural on the back burner as well as a less widely 'realized show'.
In honesty, I'm unsure if I'd recognize a cosplay as Lilith even as a fan. Also, because she changes forms so often (as you mentioned), it might be hard to pin point what you'd be cosplaying as just from seeing you.
On the bright side, the downside to that character can also be a strength. Because she switches 'bodies' so often, you don't have to worry about looking exactly like the character you're trying to portray. If you could pull off her insanely creepy attitude- and perhaps drag around a few 'loving hostages', it may be more recognizable.
Judging from the pictures I've seen from your profile- you seem like you could pull off the child-like features (which is what seems to be her trademark).
I've thought about cosplaying someone from Supernatural in the past, but I have doubts in my ability to do so. It's much harder, I find, to emulate someone that already exists if you don't already look similar (whereas what's animated can be better forced).