The reason Kirby is where he is on the tier list, and why so many people disagree with his placement is because Kirby is a counter pick character. Think about it like this: you watch Kirby play against Fox, Shiek, Bowser, or Diddy Kong, you think to yourself "dang this is a pretty underrated character." But watch him play against Cloud, Luigi, Rosalina, or Sonic, you think "what a low tier character." I think what's going on here is people confusing potential in the tier list. Tier lists are based off of what characters can excel the best at a top level on their own. Unfortunately, Kirby is not one of those characters. A tier list will tell you how good a character is while being solo maimed of you will. Yes, I believe that Kirby is a great character with an extended combo game, and great off stage work, that being in both edge guarding and recovery, but solo maiming him is very difficult. Do I believe that a Kirby will soon make top 16 or even top 8 of a major tournament? Of course. But in order for that to happen, it would be a player who mains both Kirby and another character.
If you are confused and/or not convinced, think about this hypothetical example. Let's say all the sudden, somebody discovers that Jigglypuff has a 100% guaranteed Zero-to-Death on only Mario and Diddy Kong. Jigglypuff will probably barely move up on the tier list, staying a bottom tier character, but she will still see tournament usage as a counter pick to Mario and Diddy. People may think she's underrated because she can take down these top tiers, but as a solo main, she'll still get bodied by every other character.