It gets harder and more fun for sure, but the core gameplay doesn't ever change or anything like that. It's weirdly tutorial heavy and slow at the beginning, the first pikmin game gives you like 10 minutes of tutorial and then is just like "ok, good luck."
The post game gets harder, but pikmin 4 is probably the easiest overall pikmin game, because you're just given so many tools to deal with everything. I'd say pikmin 1 and 2 are the best if you want more of a challenge, though they are more frustrating as well.
You're still gonna get Tutorialized™ every time there's a new type of Pikmin or new mechanic, but eventually, it runs out of things to introduce. Everything after that is unfiltered good stuff. There are some tough challenges, but as always, the greatest challenge is the knowledge that you can probably just barely fit one more thing in today if you time everything juuuust right