Lotta people going "as someone who doesn't really play Pokémon, Incineroar is fun to play as and is way more interesting than other potential picks from Sun and Moon," other people are going "as someone who plays lots of Pokémon, I don't like / I hate Incineroar, I wanted someone else and their design is ugly."
As a big Pokémon fan who wanted Rowlet (not Decidueye), I was admittedly disappointed at first when Sakurai went with Incineroar because the previous two Pokémon newcomers were both the most popular Pokémon from their respective games, and the "streak" was broken.
Of course, it was never really a streak, Lucario was chosen because some of Sakurai's team really wanted to put them in Brawl and they eventually figured out a way to do it (a similar story is told by Sakurai for Bowser Jr. in Smash 4), and Greninja was chosen because Sakurai liked their design the best from Game Freak's resources, especially since at the time X and Y weren't even out yet, much less proven that Greninja really was the most popular gen 6 Pokémon. I should've seen this coming.
That said, my unpopular opinion regarding Incineroar is that he sorta makes sense even taking into consideration that Rowlet is by far more popular: Incineroar is your final opponent if you chose Rowlet, so it's kinda like the final boss of Sun and Moon is playable. Pretty neat way of thinking about it but that's just me.