Opera is a complex and demanding form of music. It makes no sense for an individual who doesn’t really understand it to offer commentary or opinions. It would be like somebody who has never cared about rock and roll pontificating on the difference between a great guitar player and a hack. It’s easy for a person who has lived with that form of music. To somebody who hasn’t, it’s not.
It’s a major form of music that can’t be ignored by a site such as this. That said, the only indicator I have of a singer’s quality is what others say and write. Callas seems to be at the top of the list.