It is safe to assume that Popeye is not popular with children nowadays, as the character has been out of the spotlight for years. I think Paw Patrol and Peppa Pig are more popular with children than Popeye. I didn't even know there are new Popeye cartoons, and that article was from 9 months ago.
As for how to check what is popular, I just see the number of subscriptions of a YouTube channel, the number of views, and Google Trends. It is imperfect, but it provides a rough idea what people are searching for.