Can Canadian citizens become American citizens?

First of all, you just got a maple leaf card and haven't been naturalized, so you only have permanent resident status in Canada, not a Canadian citizen. If you apply for immigration to the United States, your nationality is still your original (China) nationality.

Regardless of the above, Canadian citizens can certainly become American citizens (my own experience). However, the timing of applying for immigration to the United States depends on the place of birth and has nothing to do with nationality. So even if you get Canadian citizenship, you can't speed up your immigration to the United States.