staying positive with full feeding team?

You don't really need to "stay positive" and fake enjoying it, it's understandable to be upset about other players on your team doing poorly (whether intentionally or otherwise). All you need to do is refrain from adding more toxicity to it, which is considerably easier; I find muting players to be the most effective way to not engage them in chat, as when I know I can't get a verbal response I feel less inclined to say something to them (and the obvious effect that they can't engage you in chat either).

I've got no problem doing this. And i'm just a man.

I don't like sugar coating my frustrations. Even if it's just mild, I'll reflect some of my frustration through mild complaints, nothing heavy handed or offensive (though some seem to take it that way, since reasons). I am all for the optimism method, by all means, if it works for whoever does it, keep on doing it. But I can't stand pretending there isn't a problem, when that problem's going to snowball a 30-40 min match into a seemingly unnecessary loss. So yes, to be accurate, I will be positive at first, but will complain if necessary.

The thing I try to do is give suggestions when I notice a pattern that puts them in harms way, for example, if I see them constantly pushing out and getting ganked, I suggest they sacrifice the gold of some minions to let the enemy minion wave push closer to our towers and then farm safely. If they're getting tower dove, depending on the situation and feasibility, I'll suggest a lane swap and see if they would prefer to farm against someone who isn't as fed as their current lane, though since I'm a mid and sup main, that only happens when I'm mid.

Outside of that, I just don't say anything at all to them.