The general nostalgia that comes with gen 1 Pokemon involves experiences you had as a kid that makes the Pokemon seem more special, because childhood stuff like that can't be replaced.
I don't know why it's less likely. Muk is a simplistic, purple goop in design. Because of it's simple design, it can be linked to about anything you want. Link it to the grape smoothies the friendly man that owned Bob's Smoothie Shack down the street used to make. Those delicious grape smoothies you got as a child every day after school, even in the winter. It's still just a purple blob.
Klefki on the other hand is more difficult to link to. Why? Because it is more creative. Because it's appearance is more complex and original, it is harder to link it to nostalgic parts of your life, particularly your childhood. What are you going to associate Klefki with? The car keys your father lost every morning before he went to work? That doesn't have the same effect. Unintentionally, Gamefreak designed Muk so shapelessly that nowadays, it is worshipped as one of the most creative Pokemon of all time, simply because it reminds you of Bob's grape smoothies.
It's extremely unlikely Muk has any sort of strong influence. It is a living sack of sludge and nothing more. The only thing that separates it from Garbodor is that Garbodor actually has a creative design (yes I just said that). Muk has a mysterious nostalgic effect, but deep down it is just sludge.