Apple smart case

Hello,

Have you tried the following:

Check the charge

As you attach your iPhone to your case, a charging status appears on the Lock screen. The status also appears when you connect the case to power or disconnect from power. If you already attached your iPhone to your case, look for the charging status in Notification Center on your iPhone.

Lock screen

Notification Center

When your iPhone isn't in the case, you'll see a green status light when the case is fully charged.

More info here:

Charge your iPhone with your Smart Battery Case - Apple Support

I don't believe it's possible to keep the charge status on the lock screen as you are saying.

You can use the Battery Widget to quickly glance at the charge status as shown in the second picture.

You can also leave feedback to Apple using the following link:

Product Feedback - Apple

You're welcome, definitely let us know if you have any other questions.

You can always leave feedback to Apple with the link above.