How to show that a website is objectively bad?

So I've been tasked for finding some sources on why our current website is poorly designed.

The site is currently, in my opinion, in a raw ugly form, though I'm no UI expert myself.

I'm using google's material design library for angular so it's at least consistent in it's look and feel.

That said, our executive team needs sources on why it's bad. I've sourced google material guidelines as one, I was curious if there was another good "Source of truth" on UI design that would allow me to pull out some quotes to demonstrate the poor design of the current site.