I don't think the dominance of English has much to do with the quality of English or American cartoons or their wider culture. Even ignoring the British Empire, the USA has been one of the largest ever continental empires for 200 years and is now a quasi hegemonic global empire with around 600 overseas military bases and the ability and willingness to launch military strikes and enforce it's will across large swathes of said empire. Knowing the language of your overlords has always been a good idea and English is now even more widespread as a second language than when the British were stomping around engaging in the odd genocide and searching for a decent meal. I suspect that even if Germany produced the most amazing cartoons in the world only a tiny fraction of people would ever to learn it compared to English. That's what dubbing and subtitles are for.

Having said that, between roughly 1850 and the end of the second World War it was considered necessary for engineers and scientists in the UK and USA at least to know German because of the German's achievements in science and technology (especially chemistry) and the political and economic power of the various central European states and empires controlled by German speaking elites. Gaining a competitive advantage over your peers and expanding your knowledge has always been a good incentive to learn another language independent of wanting to learn what your occupiers are saying.