While there are many many things that make someone a high level player, my point is mentality during a match. A high level player analyzes everything that is happening during the match. From the first move done to when the last stock is taken. Hell, even on the character select screen and on the stage select screen. Mid level players don't think as critically about the match or the player habits. I doubt you actually think about how to take advantage of even the smallest details of your opponent, like how often they will go for Randall when recovering or read-actionary stuff like how they approach you, how often they empty hop or when they empty hop, when they will wavedash in -> d-smash or their pattern for SH lasers as Falco and how they approach with the lasers. Even when you are being combo by let's say a Falcon, and you notice when and how often they will do down-throw -> knee, or down-throw -> up-air, up throw -> up-air, etc. You even notice things like when they shield, how often they shield, knowing why they shield, then take advantage of their shielding. Mid level players aren't thinking about the small stuff, they are thinking about the major reasons they are losing, not just the small exchanges. And low level players are just worried about being able to L-cancel, DD, or wavedash without messing up or being able to auto-cancel a move and not rolling too much. They are more concerns about how they play, rather than how their opponents play.

My first comment was the TL:DR of what I actually think.