I don't think people complain about Fox and Falco because they're boring characters, but most likely because they're the most common characters. I don't watch enough Melee tournaments to back up that claim, but TBH4 was quite varied and was far more exciting than if Top 8 was exclusively Foxes and Falcos.Rest of topic: people will complain about any character with significant strength or popularity. When Tyrant and Jtails were fighting people were booing because of how boring Diddy is to watch. I was thinking, what, Diddy, boring? Who is "fun" then? It's kind of like how Melee viewers often complain about Fox and Falco even though Fox and Falco are quite fast and super fun to watch. Must someone be playing Captain Falcon or Ganon for people to enjoy the show? It's the same deal as underdog cheering: when m2k is the best and winning tourneys left and right, no one wants him to win, but when he hasn't won a tournament in a while, everyone wants him to win. Just the nature of the competitive beast.
If people find Diddy boring, the problem probably isn't on Diddy's side.
Regardless, variety is also a strong factor to consider when talking about viewer entertainment.