Where is "mkpasswd" for OSX?
There's a Mac implementation of mkpasswd on GitHub - https://github.com/PrototypeAlex/mkpasswd

Alternatively, Keychain Access has a built-in GUI password generator
File menu > New Password Item... > [click the key icon]

drop-menu artificially offset for visibility



I also found this Apple Discussion on how to achieve it natively, though it's unfortunately above my skill-level, so I'll let you see if you think it might be suitable.