I don’t think you can rename the original folder or the sidebar favorite in Finder.
But there is a workaround:
Remove the Finder sidebar entry via right-click & “Remove from Sidebar”.
Navigate to your DEVONthink application support folder and create an alias via right-click.
Drag that alias “folder” anywhere or leave it in its place.
I dragged it to the desktop for easy file dropping, renamed it and gave it a nice icon.
Whatever you do with this alias, it can be renamed and can also be dragged into the sidebar.
You can save files directly into it as expected.
But you can not drag existing files into it in the Finder sidebar. As long as you use the alias on the desktop (or any other location), dragging files into it works.
So it seems you have to decide between using the original name Inbox for the Finder sidebar item, or using the alias workaround that can be renamed, but can’t receive dragged files in the Finder sidebar location, yet it can anywhere else.