I know people here are talking about anime tradition, and rightfully so, but I think characters in Ponyo look more Western than in any other Ghibli film.
"I don't want a bloody avatar!" -paraphrased from BQQ's annoyances with IMDb's stupidity

The simple answer because we force our world view onto other cultures so characters that arent grossly exaggerated stereotypes seem white to us. The characters are asian, look asian, in every way. Look at cosplay, someone once said to me, and see that the cosplayers who can play these caharcters are all japanese. White people do not usually have green, blue, purple or pink hair, but still we think of every character as white. We percieve every character that looks non stereotypical as white. I am not dissin you we do that too. But look at elmer fudd he does not look like a white person he looks like a cartoon. But we bring our ethnic superiority intooour view of him.

Ssuke and his mother looked asian to me. The epicanthic eye fold is not an issue in my opinion and just nitpicking.

and the sea father had a big nose, i just realized that i can't think of an eastern asian with big nose!

I don't know about that, the only character that looked distinctly Caucasian to me was the mother in the sea. The rest of them looked like anime characters usually do.

Three of my friends are Asian (two Japanese) and they tell me that the characters in these movies look Asian to them.

TBH, to me it seems there is a definite attempt to look less "Japanese", which in many ways, is to increase appeal. The percentage of (full-blooded) Japanese with large, expressive eyes are in a distinct minority.
It is ironic that many of the villains can be quickly identified by their unusually small eyes.
Not to sound derogatory (I'm Asian as well), but Japanese people just have a tendency for small eyes, flat-ish faces and being on the short side (relatively speaking).
Many successful Japanese models are of mixed heritage, but it seems the new generation of Japanese kids are taller than ever.

True, I've never actually head of "Caucasia" either haha

Out of curiosity just what is a Caucasian anyway?
Everybody in this film looks to me much as any person I`ve ever met or seen who happens to be Japanese.
"Any plan that involves losing your hat is a BAD plan."
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It's a long tradition in Japanese anime for characters to appear as what we might consider to be "caucasian". Anime (and western animation) allows animators to create all sorts of weird and wonderful realities, so we shouldn't really be surprised that characters in anime don't conform to our stereotypes of Japanese people. Does this mean that they are trying to create characters that look caucasian? Sometimes yes, sometimes no. Howl's Moving Castle appears to be set in a western European setting Spirited Away is set in Japan but with other Asian influences - I notice one scene has Balinese type images on the wall of the room and balinese type gamelan music.
Consider genetically Asian people who change their appearance - dye their hair, wear contact lenses. Is this because they want to "look caucasian"? Sometimes yes, sometimes no. Is a caucasian person who dyes their hair black trying to look "Asian"?

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Many Anime' characters have a Caucasian look to them. I don't think it's done intentionally, that's just the style of the animation.

Miyazaki makes his movies exclusively for Japanese audiences. The audience perceives the characters as Japanese. It does seem odd at first but ask yourself this: what ethnicity is Bart Simpson?