Does iOS 12.5.3 improves my iPhone?

The iOS 12.5.3 update provides a number of security updates as described in the support document that you arrived here from.

Whether or not it would make your iPhone "work better" would be subjective by what you would consider "better." Just so you are aware, 12.5.5 is currently the latest version in this series ... and it was released in September 2021.

Apple recommends most updates for all users because they contain important security updates that protect your phone from known threats.

All iOS updates might contain: bug corrections, new features, security patches.

I don't see any good reason why you shouldn't update your iPhone to the latest available/supported version.

Regards

Giulio