If you don't yet have a sophisticated product marketing function at your company just yet, what types of goals/key results should your PMM be held do?

Normal caveat of "it depends." 

But generally, if you're hiring your first PMM, focus them on becoming your audience expert. I think a lot of folks tend to focus too severely on ensuring PMMs are product experts in the first 90. "HOW CAN THEY TALK COMPETENTLY ABOUT THE PRODUCT IF THEY DON'T KNOW HOW IT WORKS?!"

Personally, I don't care how someone who isn't my target audience experiences the product. I don't care what a PMM thinks about onboarding. I don't care if they know how to demo it. At least not in the first 90, becuase I have other people who can do that right now, in engineering, sales, and product teams, and honestly do a better job of it.

What's more valuable, and a gap you likely haven't filled through those other teams, is expertise in your audience—who you've reached today, and who you want to reach tomorrow. Put your new PMM on customer calls, have them send out surveys, talk to analysts, talk to partners. Have them write case studies.

And THEN have them use those insights, directly from the mouths of customers, to improve how the product is messaged, demoed, and eventually—built.