I have a spoiler on my 02 and it is just purely cosmetic. what I found to help reduce the dirt on the rear hatch and glass was a set of splash guards on the rear. I still get dirt on the rear but it's substantially less.
I did find this product online (I do not have it) Its a deflector. Here's the Link
2001-2006 PT Cruiser Rear Air Deflector
I hope this helps.