I am kind of back and forth on the nerd/buff.
In general, I would always say to buff lower tiers a bit to make them more competitive to use. I am not a fan of nerfs unless it seems a great majority of people use that one or two characters to secure wins because it is more consistent. Diddy seems to be the most attacked player because of the banana or HooHah thing. I believe this takes skill to time and pull off consistently, as do other moves out there.
However, and I don't have data for or against this, if I took 50 people of equal skill, and had them train 8 hours a day on one character and I took another 50 to practice just Diddy, and most Diddy players win most matches, then it needs to be balanced.
At the end of the day, IMO, a player should be able to take any character and convert to wins based on their skill level, not because a character is easier to win with. Don't know if that all made sense. Just some thoughts.