Is it possible to mirror my MacBookPro's screen to my iMac?
Unfortunately target display mode was removed starting with your iMac model.

You could use the built in screen sharing function if your computers are on the same network.

On MacBook
System preferences -> sharing -> check of “screen sharing”

On iMac
- Open finder
- Under network in your left sidebar, you should see your MacBook
- Click on this
- Just under the top menu bar you should see “share screen”
- Click this, and a prompt will ask you for a username and password
- Enter credentials for your MacBook use account
Viola! Latency should be pretty minimal for this, and you can even control your MacBook this way!