According to another forum post, quote: „macOS and iOS don't need any extra security like defender or virus scannners. These programs will interfere with macOS'es and iOS'ses built in security. They will do nothing extra, only slow down your devices. I reccommend (sic) against using any external anti virus software on macOS and iOS devices“, you will not need any additional security software, including Microsoft Defender (for Mac) on any Mac, MacBook, iPhone or iPad.
Opposite to this, Forbes-Author Barry Collins wrote in an Article, „Microsoft Defender For Mac: Is This Some Kind Of Joke?“, that he tried Microsoft Defender and it would not extensively consume system resources, and when running a system-wide scan, it would have detected two Trojans in his E-Mail application and put them into quarantine. Yet, in most forum posts users claim that Microsoft Defender would make absolutely no sense and may even be harmful. So, I’d suggest the following: if you do not plan to use your Mac in kinda risky situations, download malicious files or use suspicious links, you will not need to install Microsoft Defender to protect you, as it doesn’t even have an e-mail filter.
Stay safe; hope I could give you some helpful advice