I'm getting a bit bored with web development as it's what I do 5 days a week so I'm thinking of learning something completely different just for fun. Is learning ios development a good idea? I not planning to ever do it seriously or as a career

Don't waste your time and efforts on learning something like a whole avenue of development unless you plan on making it a part of your skill set.

If you want to learn it because it interests you and you are curious about it as a job go for it.

Early in my career I learned WordPress development because it seemed fun. Eventually I had to do it as a job, not so fun. Since I devoted so much time to learning the framework my resume was weaker and I had to provide it as a skill to show "experience". I fucking hated being a WP admin and my "fun" quickly became a sense of overbearing dread.

Or don't listen to me, I'm not your dad.

Stay away from ios mobile dev. The IDE is awful, the review process is horribly long.
I'd go for Android mobile dev if it's mobile you're interested in, at least for a first try.

Webdev and python.
Go a bit more different with Haskell or F#.
(for no particular reason)

Or you can pick something useful like knitting or something. Not a lot of money there but surely is more useful than writing code for useless apps.

Play around with data analysis. ML stuff maybe.

Yes, I absolutely recommend iOS because of its great SDK and Swift. It’s a joy to work with. Especially SwiftUI.
The IDE Xcode is great despite of much hate that it's getting.
Yes, the Apple reviews are long (a few days) but that doesn’t really matter that much.