U-lock mount bracket on carbon frame?
The problem is not the stress from the weight of your U-lock.

The (true) stress comes from the compressional hoop stress of the bracket to the carbon fiber tube. The bracket stays in place thanks to the friction, which requires certain amount of compression. 

Unfortunately, this hoop compression coincidentally directs at the weakest point of this (tubular) design. Buckling will appear over time if enough compression is applied.

It is hard to say whether a simple bracket would cause the bike frame to fail. It depends on the user (over-torque), the design of the bracket (causing notch/localized stress on the frame), and how the frame was design (strength of carbon fiber in certain direction), etc.

There are many example of this failure type, one of which is the failure of carbon seat post. Usually, the bike frame (clamping type) was not designed to accomodate carbon seat post and cause large compressional hoop stress to the seatpost. That is why the seatpost often fail, not because of the product quality, but often the mis-use from the user.

Ref:
Compressive failure in carbon composite, N.A. Fleck (Cambridge, Advances in Applied Mechanics Vol 33, 1997)