Just look at Ditto. I don't think Ditto needs an explanation to why it is cute. Just a purple happy blob.
Clefairy looks very cute, but Clefairy doesn't look too unique which is why it falls toward the bottom of the list.
Oddish looks so cute and tiny. It's little feet make it even more adorable.
Chansey looks so huggable. It's personality makes it even cuter because it shares its egg if it sees an injured pokemon.
I think it's the big eyes that makes Jigglypuff so cute.
Somehow Dragonair holds an amazing balance of being adorable and looking regal.
Snorlax looks so cuddly and happy. I would definitely want a big stuffed animal version of Snorlax.
I remember watching a pokemon episode and I remember Psyduck being so so adorable.
Just like Dragonair, Vulpix remains a majestic creature while also being extremely cute. It's evolved version, ninetails, becomes more majestic, however, is less adorable.
I just think Eevee is one of the best pokemon because it can evolve into many different forms. And all of them are cute and sweet.