Normals has its own MMR for the que and 0 restrictions on Tiers grouping. Its never going to be great or overly competitive.
Effectively anything in the range it pulls from is fair game and it does not look at any values or stats other then generated MMR. I assume that Riot likely has their Que set to priorities low wait Time to an extent it affects quality. That's just an opinion of mine and may be wrong.
https://nexus.leagueoflegends.com/en-us/2018/02/dev-matchmaking-real-talk/
its not something that's necessary broken as MMR is a law of large #s style system but it is at times incredibly frustrating.
When they call Normals a more casual experience they mean it. Sadly Ranked is also a pretty casual experience now because of the # of players who treat it like one.
Just have to roll with the punches and accept trash games happen. If you ever played Dota allstars your more then used to this.