I see two potential problems you could encounter with that.
First is lack of a ground. This mount is intended to be mounted to a metal surface grounded to the chassis. If you haven't connected a ground strap from the chassis to one of the screws on the mount you probably are not getting good SWR or radio performance.
The second is the strength of the license plate frame itself, which is made of plastic. Unless you add some metal for additional strength, as chidychi did, the plastic will stress and eventually break.
A better solution IMO would be to use the same mount you show but actually mount it to the tub, with a piece of steel or aluminum stock inside the tub to provide additional strength.
In my case I'd have a third concern, and that is how the license plate bracket itself sticks-out past the side of the Jeep. I've had rocks and trees reach out and grab my license plate on several occasions. I gave up trying to duct tape and JB Weld the thing back together and just moved it to my spare tire.
I solved the rear window interference problem by taking my Teraflex antenna mount and bolting it to the lower-right corner of my swing-out tire carrier. Obviously this would be a more expensive solution if you don't already have a swing-out tire carrier.