"Overmuch" is not a common word (most people would just use "too much") but it is also incorrect, as "much" refers to mass nouns, and air flights are countable. I guess there may be a word "overmany" but I've never heard it! "Excess" means above a certain value - for example, "excess baggage" is baggage above the standard weight allowance. On the other hand "excessive" means more than a normal or generally acceptable amount or value, which is basically what you want here, I think. So "excessive air flights" is my suggestion.

I agree with everyone else, "overmuch" is a really unusual word. I don't think I've ever used it in speech or in writing. You can say: The air pollution caused by excessive air flights is becoming more severe. Also, do you mind if I try to make the sentence even better? "Air flights" seems a little weird and not quite natural. When I google the phrase "air flights" I get 2.6 million results, which is a lot. But "air travel" gets 50 million. Also, you probably don't need to use "air" twice in the sentence. So I would say: "The pollution caused by excessive air travel is becoming more severe." I hope this helps!

Another option: caused by " an excess of " air flights

thank you all~ your answers help~

Both are incorrect. Use "increasing".