There is absolutely no chance that pre patch Diddy was better than Brawl MK. Like, at all.
Diddy was soundly the best character in the game but he wasn't obviously the best, arguably the best, or well above average in every important attribute that makes a character great in top level smash.
MK was had the best range to speed ratio out of anyone bar none (no, sorry marth) and it was all savagely disjointed. He also had top tier mobility. He could invalidate half the cast by getting one hit and walling while running away for 8 minutes due to his unbelievable sword normals and speed.
He had the best damage output in the game, IMO slightly edging out a properly played Diddy. He comboed and juggled people for days and made people free at disadvantage due to his broken range and frame data. He could start a long string after like any whiff against the majority of the cast due to his speed and his normals did way too much damage per hit. Nair may have been glitched making it hit too many times like Peach's dsmash in melee.
His killing power was only above average, but it became insane anyway because of his damage output and how easily he could land his kill moves. Dsmash and shuttle loop could be whiffed repeatedly with no real punishment because of how safe they were. If Shiek's vanish had like 2 frame startup it would be like Brawl MK shuttle loop.
He had the best recovery and edge guarding in the game bar none. He could comfortably follow people out to the blast zones hitting them the whole time and make it back with like two jumps left easy. And if you didn't TKO him he would be coming back for sure because he had the best recovery and trying to edgeguard him air to air was suicide for pretty much the entire cast.
While MK was pretty light he had the best momentum cancel in the game bar none, which made his survivability cleanly above average when combined with his broken recovery. When you saw people mashing uair as MK after getting hit, it's because this allowed him to live far longer than was ever intended.
MK's normals were all ridiculously safe and unpunishable by the majority of the cast. Dair, Dtilt and shuttle loop were completely busted in particular as far as safety goes, but his whole kit was very hard to punish when spaced correctly. A good brawl MK knows how to play without ever taking a real risk. If Diddy had a ranged disjoint like his fair on like, every single one of his moves with the same speed, it would be closer to Brawl MK.
Brawl Tornado and Shuttle Loop>Smash 4 Bananas and monkey flip. Bananas are amazing but they have pitiful durability and can be taken away and used against Diddy by other players who have item fundamentals. Monkey flip is much less exploitable but nowhere near as safe and rewarding as Brawl MK tornado and shuttle loop. And oh yeah, if there wasn't a destiny cape clause MK would've had 9-1 matchups across the board and that's being conservative lol.
Things like MK planking and scrooging (repeatedly flying back and forth ledge to ledge under the stage as a form of stalling) was actually so flagrantly OP that we had to either address or outright ban these strategies to keep everyone from going back to melee while laughing. Scrooging was commonly banned and MK was commonly given an individual rule which would DQ the player if MK touched the ledge too many times. Which didn't even work, because when you have to start arbitrarily limiting a character like that because hes that overpowered people end up thinking the game is trash anyway.
MK, IC wobbling, Snake and to a lesser extent Diddy being great characters due to a bunch of glitches and exploits, and planking in general are the reasons why I look back on that game and have to admit it was trash as much as I loved it. And MK was at least 50% of this. But hey, they cleaned that **** up. Smash 4 is incredible!
TLDR: Lingering Brawl MK salt. If MK never happened and nothing else had changed Brawl would be have been much, much more balanced game overall and not so many competitive players would've seen it as a joke and fragmented the smash community by ignoring Brawl.