Is joining a startup a good idea if you want to start your own?
Yes — but let me add a caveat.
The further you are from “management” in start-up, the fewer skills you will really learn to start your own start-up.
That’s still OK. But if you can’t join as a Director or VP, and you really want to learn how a start-up works … then you want to be one of the First 20 employees, ideally. The First 20 all still are part of the core team, working directly with at least one of the founders. Experiencing the business and corporate issues, and all the technical issues (not just some), and all the customer issues — first hand.
After that, you will be too far from removed from the founders if you aren’t in management.