To me, "fluctuated" carries a very definite feeling of irregular movement both up and down. The idea of "up AND down" is very strong. So I don't believe something can "fluctuate down," I would phrase this as "the proportion of the audience fluctuated around a gradual downward trend..."
"Proportion of audience" is wrong. If we are talking about a television audience choosing between, let's say, election news and a crime drama, the specific phrases that apply are "audience share" or "rating." ("Share" means the relative proportion, a percentage; "rating" is a measure of the absolute number of people watching).
In any case, "proportion of audience" is grammatically wrong in itself. One can say "the proportion of the audience" or, more naturally, "the percentage of the audience." One can also say "the audience percentage."