We can't proofread your entire sentence, zhm. Can you tell us which word or phrase most concerns you?
I think if the idea is that modern-day Taiwanese are descendants of both these groups (combined), then all that's needed is the article "a" before "mixture."
If you were to omit "a mixture of," then it is conceivable that the sentence could mean that some Taiwanese are desendants of Han Chinese, while other Taiwanese are descendants of Taiwan's aboriginal population.