Is there something wrong with Poptropica? Looks like a harmless games site. If it's a misleading or spammy site I can block it.
Ads can appear for many reasons. Companies pay Google Adsense to advertise and Adsense puts ads on the sites signed up to it. For example PokemonDb is about a video game, so it's likely people here would want to play other games. Companies can also target this site specifically, though I don't know if anyone does that.
It can also be due to your browsing history, if you've visited other similar sites before or Poptropica itself. This side of online advertising is a bit stupid... I once went to a website and bought their product, then I kept seeing adverts for the product I already bought!
Also, ad blockers are nice and all for certain sites, but I make sure to only have a couple of ads per page, and always static images, no animation. So there's no real need to block ads on here IMO.