The problem with Normals is a big one, and I feel you, but it has nothing to do with the new match-making. It's a problem as old as LoL, and you could always play against players from wildly different skill brackets in Normals. And if anything, the new matchmaking should improve the situation a little, but time will tell.
The trick is, in Ranked, people play their best, and they really grind, they are always working at their peak performance or close to it, with very little variance, so MMR is an accurate metrics. For example, an ADC main won't suddenly start playing Jungle in Ranked.
On the other hand, the average performance measured by MMR in Normals has a much bigger variance. People DO swap position randomly in Normals, they DO pick champs they are bad with AND champs they are super good with, and it's super hard to evaluate properly at what level someone will play in Normals before the match starts (which is when the match-making need to do it). Not to mention that most people who are highly ranked don't play as much in Normals, so the system simply has less data to work with.
I'll take my own example to illustrate. I'm a Garen one-trick, and I'm high Gold with Garen. I played a lot of Top lane and Support in the past, so I'm worth a high Silver with pretty much any Tank/Juggernaut and a couple Enchanters. And I'm probably something along the line of a mid Silver ADC/high Bronze Assassin.
I know all of that because I see how I play, I know myself, and I can compare my performances between the different roles. And for Ranked, I do only play Garen so the system also knows that I'm a high Gold player, I'm currently Gold 2 with +21/-18 average LP gains.
But for Normals, I play a little bit of everything, and so the system considers me as an average Silver player. Which, on average, IS true. If I play Kayn jungle, and I am matched against a Silver Lee Sin, they might complain like you "I shouldn't be matched against a Gold 2 player", but I'm not Gold2 with Kayn, and the matchmaking worked correctly, nothing wrong with it. But what happens if I suddenly play Garen in Normals? The system still sees me as an average Silver player and I will stomp. And that's bad (and I try to avoid doing that, for the sake of everyone involved), but what can the poor matchmaking do about it?