I have the same problem too. I know that because the resolution of ipad is 4:3, the mirrored screen would have black bar on the left and right side because it’s 16:9 or 16:10.
But the problem is, the black bar is also appearing on the TOP and BOTTOM side of the mirrored screen. I have try to mirrored it into my monitor and TV, both have the same problem. I have tried different cable, and the problem is still there.
I also have tried every picture format on the TV. 16:9, 14:9, 4:3, overscan off, all these settings I have tried and it didn’t solve the problem.