Its not only a problem with champ level, this happens at any level, even when playing with equals and losing.
Some people just are that miserable in real life that if they can make someone feel bad in a game, it helps them feel better somehow.
It has only happened to me a few times and ive told them nicely to fuxk off.
Whenever i happen to be on the winning side of an unfairly balanced match, i never talk -Yoink- to the other player who was unfortunate enough to be mismatched by the system.
Instead, i give them advice - e.g tell them what they are doing wrong and what works better.
Being -Yoink- to new players drives them away - and you will see the player base dwindle.
In addition, i use that easy opponent to try out some new unit combos and strategies.