Just saw this:
[slayer-pantheon-popcorn]
[slayer-pantheon-popcorn]
It means that their algorithms can't predict your MMR as accurately as it can when you play solo because they don't have enough information from Riot. It doesn't mean your real MMR might not reflect your skill if that's what you're implying.
MMR values from 3rd party sites are nothing more than an estimation based on their own limited data and rough algorithms. Take them with a grain of salt.
It's an indicator that this person isn't where they are meant to be. I was playing with a 4 man premade so I checked them out on OP.gg. Two were Diamond last season, two were plat and the last was Gold last season. I checked his MMR and saw the above. He got stomped by a D1 player so bad that it took a whole hour to finally win. If this was last season, I won't have to worry about inexperienced players ruinning my game (Contrary to popular beliefs, boosted players were rare in Diamond).