Is Malwarebytes useful for protecting a Mac?

Malwarebytes is often recommended here in the community as a useful tool for doing an occasional manual scan of your Mac when you suspect an unintended installation of some form of malware. It's generally a lightweight app when used in that context.

However, the use of Malwarebyte's always-on scanning feature is not recommended. As with any anti-virus, cleaning, security or VPN apps, there is some potential for conflict with the Mac's own built-in security that can cause performance issues.

Your Mac requires no always-on, third-party anti-virus or anti-malware software.

Apple has built these protections into macOS and they are updated regularly.

• macOS security built right in. - Apple Support   

• App Security Overview - Apple Support

Not really necessary. macOS is very good at protecting itself these days. FWIW though I used Malwarebytes for years and it never once detected an actual virus or malware. However that might have because I use common sense when online. Actual viruses that spread without user input or knowledge are basically a thing of the past. Bad actors use social engineering to get you by tricking you into providing personal data. It’s called ‘phishing’ whereby they dangle bait in from of you hoping you will bite, the old tried and true “if it sounds too good to be true, watch out”. Human greed (something for nothing), fear (the IRS is after you, your social security has been locked, etc), of course lack of common sense are the biggest baits.

Sorry for the lecture but you’d be wasting your money on any anti-virus software for macOS.

It's unnecessary. As are all third-party Antivirus apps.

Just to add, follow these simple rules…

Effective defenses against malware and ot… - Apple Community

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support