It is easily readable by the iPhone camera app. The link for the QR code popped up right away.
I mean for business verifying the vaccine certificate. I did a non-scientific test, with my Apple Watch displaying the SHR QR code, using
Vaxicode Vérif (required by the Government of Québec) and
Verify Ontario (required by the Government of Ontario).
I conclude that capability of scanning such a small QR code depends on how the developer implemented it. It took me about 1 minute on average to scan successfully the code using Québec's app, and 15 seconds using Ontario's. I tried 3 times.