I think the important thing is to find a balance that satisfies you. If you never do any self improvement, you might be missing out on a much better life that would cost you relatively little to get to. But maybe your current life is hunky-dory, in which case, that's great, you do you!

And over-focus on "self-improvement" may mean you miss out on things along the way. Making yourself miss those things may be more of a decline than improvement for you.