Is Drain Crown on Dragapult worth it?

I'd say it's always a matter of how you play/play style, your team and your mastery level of the game and that specific Pokemon.

For example, if you solo queue (even in ranked), you shouldn't blindly follow top players builds... they not only know exactly what they are doing, but they also have well established and organized teams to support them.

So Drain Crown is okay, but I don't use it... I prefer a more balanced build... my advice is to test it and decide which you prefer.