Lady B

I know google ads can do some malicious things occasionally, and I remember a specific incident with malicious software being auto-downloaded from an ad on Neopets, either last year or the year before. I recommend installing AdBlock. If that doesn't stop the pop-up, it's possible you have some invasive malware (in which case you might want to get your computer checked out). If you're only getting this pop-up on Neopets, and no other websites, it's likely a malware-infested ad, though.
 
If you want to report it, you might be able to report it to the site's admins through the abuse report form and they might look into it. It could lead to the removal of the ad, if that's the cause. I'm honestly not sure how they handle reports like this.