That list was supposed to be people's 20 favorite animals. Not many people have monkeys as their favorite thing. Monkeys and birds are not typically people's favorite animals, but they are super underrepresented. They are still extremely important for real-life zoos, and with the ability to create your own climbing frames, primates and other arboreal animals are fun to build for in Planet Zoo. Not every new animal is going to be a crowd-pleaser. Still, I think there are some that would do very well in their categories.
For birds, I think a colorful duck would go over well, especially given how people liked the swan. The Mandarin duck is an obvious choice for a crowd-pleaser, and it would be popular here as well. A few more species would allow people to create duck ponds and fill out habitats from other regions. One of the pink trio (scarlet ibis, roseate spoonbill, or American flamingo) would add something colorful for both Americas, which currently need birds more than any other region. The secretary bird has a ton of fans, partly from ZT2. I think it would do awesome with a general audience, but a grey-crowned crane would also be appreciated and possibly more useful with interspecies enrichment bonuses. Africa has a ton of popular and great ones. A pelican would probably do ok, especially if it had interspecies enrichment with the pinnipeds. People have heard of them, and they are funky in a good way. Domestic fowl (chickens, turkeys, and guineafowl) would likely be well-received among the options for a domestic pack. The problem with most birds in Planet Zoo is they reproduce too quickly and then get sick. They are a bit of a pain to play with. I would imagine most people would have ~2 in their top 20, a favorite ratite, and a flashy one like the swan, peafowl, or flamingo. They aren't cute and fluffy like red pandas or large and impressive like elephants, but they make great choices for smaller habitats, and birds with interspecies enrichment are fun to spice up larger habitats.
For primates, I think tamarins would be the best-received of anything. They are colorful, cute, endangered, and common in zoos worldwide. Squirrel monkeys are also small, cute, and colorful. I agree that spider monkeys would be interesting to watch kind of like the gibbons. Add in the color morphs, and I think they would be received ok. Flashy African monkeys like the guereza and/or De Brazza's guenon would probably do ok. Golden Snub-Nosed Monkeys are pretty popular animals in China, which has more of the playerbase than any other country. If they were voting, I'd imagine their monkey would be decently popular. I would guess most people would have ~2 primates in their top 20, a favorite ape and a smaller one. A lot of people find primates scary, but they are interesting to watch and a major part of zoos. Building climbing frames for them is fun.
I would imagine howler monkeys, baboons, Indian monkeys, and rheas would not make many top 20 lists, but they are still very important species to flesh out the roster. Taxonomic diversity is important for the gameplay overall. Not every animal has to be a crowd-pleaser.