Be Supportive
I see people harp on others for how they play all the time, and while I know this is inevitable in games like this, I really wish it wasn't. If people play "bad" it could simply mean that they're trying out a new champ, new role, or they could just be new. And if this is the case, they should be encouraged and not berated for the missteps they make. If you see something better they could have done, then tell them calmly. Don't curse them out. I main support. Recently, I decided to try out adc for a friend of mine because they don't like laning with strangers. I thought I did pretty well considering I rarely adc but after the match he said I was a bad adc and that he'd rather have me go mid. It really hurt my confidence and it wasn't until my best friend gave me tips on how to build and laned with me that I finally tried adc again. And you know what. 16/5/21 the first game. Turns out I have a knack for adc. All I needed was a little encouragement to give myself the confidence to actually try new roles and builds. So if you see someone struggling, help them because they might turn out to really love whatever champ/lane/role that they're trying. They might not, but you'll never know until you let them try.