Is it considered okay to ask for the opinions of admission staff or committee after their decision of declination? If one applies to a graduate school and gets declined, is it considered okay to ask for the opinions of admission staff or committee, about their thoughts on the person's application package (gpa, experience, personal statement, recommendation letters), for better adjusting his plan of applying to other schools and career?

Thanks!
There is nothing wrong with asking, but don't expect an answer that is particularly helpful (although you may get very helpful answers). You may get a standard response that doesn't tell you much, and I would think it pretty rare for someone to take the time to craft a detailed response.

If you do ask, make sure you do it in a professional manner, with a couple of key questions about particular issues, like those you mentioned. I would avoid a generic email that simply asks, "Why didn't I get the job/an acceptance?" or "What didn't you like about my application?" Obviously, if you do get a response, follow up the response with a brief thank you email (and definitely don't get in an argument about the response if you don't agree with it!).