DMI stands for Deliberately Meaningless Index. It is calculated based on your player's skills and has a very large component due to game shape.
A player with high game shape will be play better - it is irrelevant that it also inflates his DMI. Having a player play 50-80 minutes eah week will get him up to strong-proficiient. Go outside that limit may no drop his game shape. Going outside that limit consistentlly (say 30 or less, or 100 or more) will definitely drop game shape.