Uuuh I'm not 100% positive cause I've only been playing ~8months, but I watch Scarra a heck of a lot and I have to say, he's super positive about everything. (AD Sona top on his team doing terribly? No worries, she's just bait so their team can gank and get repeated kills)
I do believe most pro-level streamers though are all great people. Riot wouldn't allow such a large role model in the community be toxic.
If you ever watch 2.2kish+ elo games everyone just seems so chill about everything :P
Also, I have a list of 4 awesome (and hilarious, but very true) videos about attitude in league that I showed to a er... less joyful... friend of mine a while back. I'm not at my PC atm but I can post it later