So, say my smurf is a higher rank than my main...

i think to a great degree its the way the system is implemented since if you've played a number of ranked games when you're inexperienced (bronze for example) your mmr will reflect that if you sit out of ranked and focus on improving your normal games and get to a higher level of play (we'll say plat for this) and then do a ranked game because you've obviously improved, your ranked mmr will still reflect your bronze games and you will still be matched with other bronze players for your teammates so at that point its a question of how hard you can carry the game really if the other 4 people on your team are so low mmr that you can't carry the team to victory (with your plat level game knowledge) then the lost games will reflect that in your mmr despite whether or not you personally have improved

of course i'm not 100% sure if this is how the system works so if someone wants to correct me by all means but i think this is just about how it works (and also why if you start playing at a silver level its so hard to get out of bronze because silver isn't exactly anywhere close to pro lol)