I notice there isn’t really a post that talks about what MMR is and all that here, previously all mention of that was informal small talk passed around in the old forums. Don’t worry, this should be much shorter than my last OP.
MMR is an acronym meaning “MatchMaker Rating”. This refers to the hidden elo that you gain and lose when you play matches. This is the primary factor in matchmaking within most game modes. The catch with ranked is that you also have a separate VST (Visual Skill Tier) that shows your ranked progress this season and is the primary focus of players. So often (not always) you have mismatches in VST that are accounted for by similarities in MMR. The most obvious example of a MMR to VST mismatch is with players whose VST has decayed quite significantly. They tend to be matched with players around their MMR and higher earned VST, but themselves show a significantly lower tier. It is possible for MMR to inflate dramatically but not without a severe win streak, especially initially. (Smurf detection)
Now, what’s the deal with 5v5 and all that? This is a common frustration for many players but it’s not as simple as a “bad matchmaker”. There are two important things to know here (sourced from dev posts and Nivmett):
- 5v5 Casual MMR started THE SAME for everyone
- 5v5 Ranked MMR takes into account both your 5v5 Casual MMR and your 3v3 Ranked MMR
Additionally, its important to remember the distribution of players. The vast majority of players are actually ranked somewhere in 4-6. 70% of players are Hotness and below.
Think about this for a second, if something hasn’t popped out at you besides “SEMC SO DUMB FIX MATCHMAKER PLS 
”.
Everyone started out on the same page with what was effectively a MMR reset for 5v5. Most players are tiers 4-6. This means that it’s completely normal to have huge variance in the visible 3v3 VST. Because everyone starts out on the same page, 5v5 Casual MMR is influenced by your performance in 5v5 matches - not 3v3. Additionally, when we go to examine 5v5 Ranked MMR, it factors in both 5v5 casual MMR and 3v3 Ranked MMR. Its likely that your VST is a tied in the same way (wouldn’t make sense otherwise) - most but not all higher tier players I knew seemed to have higher starting 5v5 VST.
The single biggest issue with all this is NOT that 5v5 Ranked MMR set everyone to 0. The issue is that people didn’t have long enough to be sorted more efficiently into approximate Skill Tier levels, MMR wise. Its the equivalent of trying to filter a bunch of gravel and only giving your sieve a few shakes and expecting the resulting mixture to be fairly uniform. So 5v5 Causal MMR wasn’t sorted out for long enough. Then its thrown right into ranked alongside 3v3 MMR which primarily showed mechanical skill. Yeah, we gonna have some major inconsistencies. And everyone started out with equal MMR and most players are 4-6 in 3v3 VST, then yeah, we’re going to have a lot of “MAtchmaker Discrepancies”.