No, it's not unreasonable to expect that. That doesn't mean it's ok to abuse the people not trying their best (i.e. failing to meet your reasonable expecation), but I don't know exactly how you handle that part.
Well I generally try to let my frustration out, but not be abusive in the process.
For example if we're getting ganked a lot, and I'm the carry, and nobody is buying wards I might say:
"Can somebody buy wards please?"
If people continue to ignore me I might say:
"We have 3 freaking supports, can one of you buy wards? Holy crap."
If the team isn't working well together or coming together for team fights I might say in a frustrated tone:
"Can we stop going off alone and doing our own thing please? We could win teamfights if we stuck together."
If someone on my team does something EXTREMELY stupid I might say,
"Are you serious dude?"
I guess my point is that I still get verbally frustrated, but I try not to single anybody out, or pick on any one person to vent my frustration on. I try to spread it out equally across my team so that nobody feels personally attacked, and in the hopes that my frustration can possibly make us work better together.
On the flip side, I also give credit where credit is due. I often say things like "Good job" and "You rock" when somebody is a really good player, or when they do something really good like get first blood or have wonderful ward placement the entire match.
So I guess what I'm saying is that I try to be somewhat fair in my praise and criticism, but even in a game where I am heavily critical, I try not to single anybody out. In the worst case scenarios I might say "This entire team sucks" then afk in base. I find myself AFKing in base a lot actually, because after 9 years I can typically tell whether a team has what it takes to win the game, and I won't waste my time if they don't. I think I'd rather AFK in base and do something else than elevate to the point that I'm verbally assaulting people though, so I think it's a good compromise.
The other wrinkle is that people who really are trying their best, but failing to meet the expectation of "play well", also catch abuse in many cases. This can dissuade them from getting enough practice such that "playing their best" generally means "playing well".
Sometimes it is hard to tell whether someone is trying their best and they're just new/bad, or whether someone just isn't taking it seriously. However, new players typically apologize when they make a mistake. Typically all it takes is one apology for me to realize the person is really trying, and from that point on I'm generally very supportive and understanding of their mistakes.
Once again, even if someone is just flat-out horrible or just having a bad game, I try really hard not to single them out and make them feel attacked. I don't know if that helps but it's the best idea I have right now.