Is it any problem for battery life while charging iphone 14 with 5W iphone 6S charger?

Whilst a 5W USB Power Adapter is fully useable with your iPhone 14 - charging will be much slower than when using a more powerful USB or USB-C Power Adapter.

Other than faster charging, there is no technical reason to require use of an alternative USB power source.

I've always done so without any issues, not with ipads though because even though they are charged a tiny bit then it does not even indication that being the case.

have you read

About the battery and

performance of iPhone 11 and later - Apple Support

and maybe

iPhone Battery and

Performance - Apple Support

and Batteries - Maximizing Performance - Apple