China is America's annoying younger brother who might have a little more athletic talent, but has no people skills; we simply tolerate China at this point. You don't put sanctions on your allies because of a place like North Korea. It's not our business who the Chinese want to deal with economically.
China will try to make friends with anyone that suits them. They would not hesitate to turn their back on an ally if they thought it would get them something they want. They do not care about American sanctions because they want to have good relations with North Korea. They cannot be trusted.
No, China is not an ally of the United States because they are too closely aligned with North Korea. They should get rid of their alliance with North Korea and prove that they have the United State's interests at heart. Otherwise, they are much more closely in line with North Korea.
China bought an incredible amount of coal from North Korea, in direct violation of international sanctions against the so-called "Hermit Kingdom." The purchase shows that China doesn't care about international guidelines - guidelines that the United States has both created and abides by. As a result, China cannot be considered an ally of the United States.