I’m getting an intermittent notification about Safari being infected on my iMac. Should I be concerned?

Could be a scam to try and get you to open or download something.

see > Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Could also be from some third party blocker or AV-ware that you added to Safari or the macOS.

For a better idea of where the message is from, run EtreCheckPro on your iMac.

https://etrecheck.com/en/index.html

Then post the report if you need help deciphering the content your EtreCheckPro report.

https://discussions.apple.com/docs/DOC-250000211

Open Safari > Settings… > Websites > Notifications and clear out the list.

FWIW, the current version of Ventura is 13.4.1 (c), you should update.