Is there a way in Mikrotik Wireless access list to say, limit connections from specific vendor MAC addresses?

Like how I can use an entry wildcard like 00:0C:42:xx:xx:xx to allow only mikrotik clients? or 00:15:6d:xx:xx:xx to allow only ubiquiti clients? etc.

Try to use "00:0C:42:00:00:00" format.