to suggest Secret Life of Pets has the same level of reception as Shrek is just ridiculous. adjust the ticket prices, Pets is just under 400M, Shrek 2 is 650M. Shrek is just another level. It's like comparing Hunger Games to Avengers.
You are correct that the majority of animated films have bigger OWs in the sequel but that's just less likely at higher openings. If a film starts off with a dampened OW then it gets watched and rewatched, then the sequel comes out and the OW is bigger - that makes sense. But when it already starts with a huge opening, it is less likely to increase.
If you look at animated films that opened above 80M adjusted:
Madagascar 2 - 80M, 2.85x
Madagascar 3 - 68M
Cars - 84M, 4.07x
Cars 2 - 75M
Alvin 2 - 91M, 2.85x / 3-day 59M, 4.41x
Alvin 3 - 27M
Ice Age 2 - 95M, 2.87x
Ice Age 3 - 82M / 3-day 51M
Monsters Inc - 101M, 4.08x
Monsters Uni - 90M
Finding Nemo - 107M, 4.83x
Finding Dory - 142M
Despicable Me 2 - 168M, 2.57x / 3-day 98M, 4.41x
Minions - 129M
Minions - 129M, 2.9x
Despicable Me 3 - 74M
Shrek 2 - 191M, 3.42x / 3-day 160M, 4.08x
Shrek 3 - 162M
Shrek 3 - 162M, 2.65x
Shrek 4 - 83M
That's all the ones I can find, and every single one of them, with the exception of Finding Nemo, decreased in OW in the sequel.