App Store App iPhone battery usage

It seems likely your App Store app is very busy performing some previously neglected application updates.

You can, in the future, check for and apply these updates manually. Updates for some (many) apps occur frequently.

How to manually update apps on your Apple device - Apple Support

Mine has been doing it since iOS 14. Note that it is using 50% to 70% of the actual usage, not 50% to 70% of battery capacity. For example, if your phone has used 30% of its energy then 50% of that would be 15% of total energy use. So if there isn’t much else using energy the App Store usage will appear to be large.

Mine has been doing this too, and it's not because of app updates — it's been doing this for the weeks since iOS 15's update. My iPhone's battery is easily halved now, which is very handicapping.