So people are getting harassed in ARAM if they don't do well with their Champion.

I know that's not a particularly insightful or new observation when it comes to LoL, but given the nature of ARAM you would expect that people would be more tolerant of others not performing so great since there's a fair chance they're playing a Champion they aren't comfortable with. Or so it would seem. (I feel it used to be like this) And I think this speaks to an elephant in the room of sorts when it comes to ARAM:

It's not really random anymore. It's abundantly clear that in almost every single ARAM match, more than half of the players (I'd say on average 55%-65%) are on their main Champion or at the very least someone they are proficient with.

This creates an atmosphere of "Why doesn't this person know anything about this Champion?" in an environment that, not too long ago, would have been a ridiculous thing to ask because it had a self-evident answer: "It's random."

But the expectation has shifted because you're now able to hedge your RNG in so many ways (Alt-account, Re-rolls, and Re-roll switching), that the community expects you to roll something you're comfortable with the vast majority of the time, and therefore it's open season on you if you under-perform. In other words, the not-so-random-anymore nature of ARAM has shifted the community's attitude to playing poorly in this mode...

...

...which really really sucks when you roll someone you aren't good at playing when everyone else is on a main/semi-main. You will more or less be the cause of the loss or a significant contributing factor in a very obvious way. It's not a good feeling.