To my perspective, I think if frustrating to play against, regardless if you stomp them or not, being unfun to play against is what I call a bad design.
And it's funny enough how certain mechanics can simply make a champion toxic even if it's meta or not. Even if it's just 1 mechanic.
However, one can argue that a champion overloaded with too any things can count as a bad design. Which I somewhat agree.
I ain't gonna give examples about which champ is which but I'll leave that to you and say if I agree on it or not. Give valid enough reasons of course.
Curious as it is, I sometimes ask myself "if a Rioter designed a champion that's very frustrating to play against, does it really mean it's a success or not? And does it also mean the champion should be removed due to customer feedback all the while the Rioter that designed it deserves to be fired solely for this?"
It's a curious topic really, would like to hear or in this case read other opinions about this.