Do East Asians look down on Southeast Asians?

Yes, but its not because of any geography related thing but rather economicallyrelated. Think about it – do Americans look down on Mexicans and South Americans? Yeah, they do. They have stereotypes about them. Why? because they are poorer nations.

Where would people rather go Europe or Africa? Who would they rather associate with? Because Europe is wealthier in general than Africa is, there’s stereotypes associated with Africans too.

Same applies to East Asia and Southeast Asia. Historically SE Asia (exempting Singapore) wasn’t as economically successful or powerful as China/Korea/Japan so they were looked down upon.

Its very economically related. Just ask Koreans (probably the most racist of all East Asians) what they think about Filipinos, Taiwanese, Malaysians and Singapore. Chances are, even though Taiwan is geographically close to Phillipines and Malaysia geographically close to Singapore, they are going to see Taiwanese/Singapore people in a better light; because of economy reasons. The light/dark skin color is just something related to this. For whatever reason in history, the economically poorer nations had people with darker skin, and the wealthy and powerful nations in history had people with lighter skin. Thus people around the world, not just East Asians, tend to look up to people with fair skin and look down on people with dark skin, but not because of skin color but because of historical economic reasons associated with that skin color.