80% Not meeting expectation, 20% Clunky Design.
The expectation was that with two quadrupedal forms, the final would be the very first actually quadrupedal fire starter. Currently only 4/21 are quadrupedal. It would have been unique.15/21 are bipedal. If the second form had been bipedal, or an in-between posture (hunched like marshstomp or sort of bagon-like attempt at being bipedal, etc) the hype wouldn't have built up so high. The actual design wouldn't need changing just the art and animation.
People loved the oil/wick gimmick of litten, the fire-hairballs. Torracat also spitting fireballs and batting at them with its paws was liked, as was its "bell" fire sack on its neck. The cuteness themes were completely dropped in the final form. There's nothing about oil, wick, hairballs, or even breathing fire, nor any of the cute cat things. I feel like if litten started with a fighting theme (self-combusting paws? Like charmander's tail or cyndaquil's back), and torracat played more on wrestling (and the bell having a collar) the transition would have been smoother to Incineroar.
The theme is fire/fighting. Not the type, but the theme. 4/7 fire starters have a fire fighting theme. While it being dark type helps a little, it's still clearly a fire/fighting theme and the fans were hoping for something new.
People were hoping for a panther, a tiger, or other beastie-type creature, as in something "fierce/cool", Incineroar is a bit "goofy/camp". Panthera, large Canid, prehistoric Predatory mammal, or other "beast" type shape would all be unique among starters, 0/21 are that way. It has a very humanoid proportion - around 7/21 have distinctly humanoid body types.
Once disappointment dies down, all that's left is a few clunky design choices.
1. Hands like sausages, limbs like ham-shaped noodles.
2. Sideburns.
3. Belt isn't actually fire, looks plastic.
