The ads on Duolingo come from industry-standard ad networks like Google and Facebook. They’re not chosen by Duolingo but by the network (so, for example, if Google knows that you like to buy My Little Pony figurines, they will choose to show you ads for that).
However, we have filters in place to block any ad content that would be considered objectionable by language learners. We take this very seriously – if you see anything remotely inappropriate, please take a screenshot, record the destination URL of the ad, and report it to abuse@duolingo.com.