Up to this date I still don't get why this duo/trio system works as it is. If a high level is willing to duo or trio with low level(s), it means that low level player is good enough for the high level player to want to play with him/her, hence they should be matched with players about as good as the high level player, not the other way around.
I remember back when I wasn't 30 yet, I kept running into silver~plat ppl on the other team and their response @post game lobby when ppl raged was "we're duo que". Like, does it make sense to you to assume the high level player most likely performs as bad as the low level player(s) so they should be matched with other low levels?
If you have a premade team with a large MMR difference (which does take summoner level and games played into account as well in Normals) it does lean somewhat towards the higher-skill player already, plus there is an adjustment for being in a premade. So if you had a duo with, say, one player at 2000 MMR and another at 1000 (which is like Plat and Bronze), the average MMR for the duo might be considered something like 1600-1700 (high Gold?). Then it will look for other groups or single players around that MMR to fill in with. Riot has a LOT of data about winrates for groups with various MMR disparities, and they have tried to adjust it so that grouped players still maintain a ~50% winrate.
I've played on teams with large skill disparities, and it usually finds matchups that feel reasonable. When it doesn't it's likely to be from factors like the high-skill player not having an accurate MMR in Normal matches, or the low-skill player being a smurf that hasn't played enough games to be properly rated.
Edit:
and like the other guy above mentioned, if you came in doing really well because you're an experienced RTS and/or MOBA player, it can adjust your MMR upwards very quickly so you're not just stomping people game after game.