Do iMac computers require malware protection?

Not beyond the defences built into the OS, and not downloading software from sources other than the App Store and the developers' websites.

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As other have already mentioned

A good defend is recognizing malware / adware before it touches the computer

Security. Built right in

Mac app security enhancements

 The Built in Security  is all that is required to protect the computer.

Protecting against malware in macOS

There is Adware & Malware which will affect the computer and is generally downloaded from “ Shady “ Websites or Developers who Inject this software into the download.

From a well written User Tip from @Kurt Lang

What is malware? - Apple Community

+1 Niel is correct. If you want a good running Mac simply keep Mac OS up-to-date and restart the computer about 1x per week. Never install third party:

     •      Antivirus Apps

       •      Cleaning Apps

       •      Security Apps

       •      VPN Apps

       •      Maintenance Apps

They already have it. You do the rest. Read Effective defenses against malware and other threats - Apple Community.