No. You need to start side car from the device you want to extend the screen from, not the destination device. You add a screen to the device you are currently working on. It makes no sense the other way, because it's an option for the device being worked on, not the device being used as the screen. If you are not working at your Mac, then adding a screen to the Mac is meaningless. you would have only 1 of the screens in your hands you Mac has access to and would be unable to see the other screen, and further would not be able to do anything on the Mac from the iPad only. SideCar does not let you control the Mac from the iPad, it lets use the iPad as a screen for the Mac.
If you want to access your Mac and to control it from your iPad rather than add a screen to the Mac, then that is not Sidecar at all. You can use apps such as Teamviewer for that.
https://www.teamviewer.com/us/