Backing up a MacBook Pro while plugged in overnight

As long as you leave the lid up on a 13-inch model for better cooling, I see no issues.

However...

Maybe the better question is, "How can I make it NOT take 9 hours?" What backup device are you using? What is the sub-model of your Macbook Pro?

it sounds like an old USB2 external drive. When I back up my wife's iPad to her USB 2 computer, it takes 70 minutes for about 70GB. WhenI back it up to a USB3 Mac, it takes 7 minutes for the same data. You can expect similar performance differences with external drives.

What you can use to improve speed depends on your Macbook Pro sub-model, which we are waiting to find out. If 2012 or newer it has at minimum USB3 ports.

Will add recommendations once we know more about your macBook Pro. Sub-model is always available from "About this Mac..." under your Apple menu. Example:

are you using Apple's built-in Time Machine?

Initial Time Machine backup is a full backup, and takes a long time. But it maintains this database...

... after the initial backup, only changed files are added to the backup SET, and previously stored files are not duplicated in the backup set.

You get access to the full complement of files as of ANY date and time for which a backup is still available. Time Machine uses Links to previously backed-up copies of the files that did not change. It manages all this automatically AND...

... since the modest incremental backups are scheduled to run automatically in the background at low priority, there is no need to set aside a separate time just for backing up. When disaster strikes, Time Machine backup is the one most likely to be current, to save you from disaster.