M2K played D3. Did pretty good, but saw people like Forte having more fun and doing better with MK. Tried out MK and made the switch over. During this time, the lamest person with MK was Teh Spammer actually. Not too long into Brawl after this point, you had Plank at a tournament grabbing the edge over and over on Battlefield vs SK92 I think. Planking was born. M2K adopts this later on to a much more significant degree (Spammer as well basically added some planking to his overall patient gameplay).
Actually come to think of it, M2K himself wasn't the originator of many lame Brawl MK tactics. He usually was the most visible MK doing them, and sometimes took them way further than they were before. Tornado spamming certain MU's may or may not have been M2K at the beginning, Planking wasn't M2K, IDC wasn't M2K, the Uair into Nado IIRC wasn't M2K but he "abused" it the most in teams. The Snake C4 free kills, aka "Omnigay", wasn't made by him either. Scrooging sort of was mainly M2K but the idea wasn't actually that new at the time (basically seen as an extention of planking).
The few things I can think of that he truly originated without debate, were some of the early MK combos and tilt tricks with Dtilt. I dunno how many people remember, but very early into Brawl M2K figured out a sorta generic combo that would work on 90% of the cast. Come in with a glide real low, hit the glide attack, then go into Nair, and then Ftilt or try for a Dtilt trip. It was an easy 30-40+% on anyone in the cast and he had it down really well. He was also one of the few people to know intimate details like the invincibility on grounded shuttle loop, and he spread a lot of that knowledge to other MK's at the time who didn't know. In fact, you could argue he single handedly helped out the MK scene the most for the things he knew and shared with other top MK players.
RANDOM BRAWL HISTORY TIME OVER