That's not true. You're stereotyping criminals. People who commit assault, petty robbery, public indecency, etc. are probably on the whole not brilliant. But how about fraud, embezzlement, or parking infractions?

Given that we're talking about cybercrime here, what are the odds that the criminals in question are too dumb to Google "how can i get around whatsapp image scanning"?

It's a general statement; there is always individual variation. It is also generally true of fraudsters and cybercriminals.

I hear this a lot, but I wonder if that is just because the only criminals you hear about are the not very smart ones doing crime on unencrypted monitored services. This sounds like a survivor bias situation. How can we know how many criminals there are if we only know about the ones we know about?