The only reason why Sun and Moon had good character interactions between Pokémon was because everyone who wasn’t Ash had so little Pokémon of course they had the space to form more interactions between them. Nobody had more than three Pokémon and that was only Kiawe. So naturally it’s easier to develop interactions and bonds between the main Pokémon when each character only has about two. The “great” character interactions between the cast Pokémon was at the expense of them not having anywhere close to a full team.
That is true but considering SM had twice the amount of trainers, that really meant the Pokemon total was no smaller than previous series where there were only three or four trainers. BW and XY's Pokemon total if anything is less, just shared between less trainers.
Also my previous issue was that many series had decent interactions to start off with but then dwindled down as the cast evolved or developed. XY was a good example of this, with a very close unit of quirky Pokemon at first that slowly became out of focus as they evolved (eg. Eevee, who pretty much never interacted with Bunnelby or even demonstrated much of a personality often after evolving into Sylveon), while SM maintained or even continued developing interactions with bigger or evolved Pokemon. It's easy to think Pokemon like Lycanroc and Turtonator would have just shifted into being satellite characters for their trainers if getting anything at all, but they kept appearing within the unit. I think the only exceptions were Charizard and Primarina, who admittedly didn't have a huge amount of personality to begin with.
It also helps that I didn't really feel a segregation between 'serious' and 'comical' Pokemon in SM which I feel often hinders development. By the time of XYZ for example we had a clear hierarchy of suave badasses like Greninja and bungling nobodies like Chespin who only took part in one particular field, while SM Pokemon tended to get the best of both worlds, Pokemon that were whimsical and took part in lots of interactions but were still allowed to be competent and effective to character development.
(I don't mean to continue the XY vs SM debate by the way, just XY tended to follow the extremity of what I'm trying to explain. Previous series had this problem as well obviously, but to more varying degrees. Heck, the fact XY had it more clear and obvious might be the reason SM tried to avoid it in the first place.)