Jin Yiang isn't Chinese-American. He's Chinese. Chinese people who were born in China have accents when speaking English. It would be ridiculous if he was Chinese and didn't have an accent. I haven't noticed Dinesh as having any accent, but if he does have one, it's the actor's actual accent. He's from Pakistan, so maybe he has a trace. Which is completely realistic and completely fair.

Having spent some time in India and Pakistan, the actor's accent (Kumail Nanjiani) is upper class/elite Pakistani English, most likely influenced by going to a good quality private school. His pronunciation of certain words is similar to the English spoken by, for example, senior Pakistani journalists on their domestic television channels or politicians in Islamabad.

Yes, Kumail Nanjiani (who plays Dinesh) grew up in Pakistan and moved to the U.S. for college. I believe Dinesh is also supposed to have grown up in Pakistan (albeit with a Hindu first name? Well, of course, even if rare, that can happen), and then immigrated to the U.S. as an adult. Hence, the accent.

I think the point is that the show doesn't seem to feature Asian-Americans, even though there are a lot of Asian-Americans in the real Silicon Valley.

Even in a restrictive sense of "Asian-American" as meaning those of Asian descent who grew up in America, there are a few presumably Asian-American minor characters on the show, perhaps most notably the Indian "brogrammer" (played by Aly Mawji; imdb is oddly inconsistent as to the character's name, switching between Aly Dutta and Naveen Dutt). But yes, more Asian-American representation would be good (in particular on a show reflecting the demographics of Silicon Valley, but also on TV in general).

Many Chinese-Americans were born in China.

There is no proper English accent, to a New Zealander, people born in California sound weird.

among those with Asian origins that I have worked with, I'd say at least half are foreign born, so I don't think its entirely unfair.