AT's require a lot of work too. A lot more than Trophies, Stickers, or Spirits. In fact, some have a huge amount of work put into them. Alucard, Bomberman, Spring Man are examples of rather robust AT's.
That's rather large representation, all things considered. They need animations, hitboxes, often hurtboxes, and they only have simply less than a typical character other than costumes. That's hardly "minor". Trophies are the most minor due to less gameplay stuff and they still have to translate into multiple different languages. All of those small things take a lot of work. In fact, music is the smallest amount of work, with only licensing require a lot.
Calling 'em minor is way downplaying the huge amount of work these things overall take. Summon characters are pretty difficult to overall make work right(there's going to be smaller ones like Goldeen who take less work than some like Meowth, obviously, but one is an attack AT while the other does no damage), and both have a lot of unique animations to take into account too. Don't forget that even characters have had glitches related to AT's, as well as the fact AT's have hitboxes that can clash with a character, and are affected by things like Counter moves. This is intentional. They're just NPC's with overall less but still major work done. There's no world where AT's are "small" by any means.