You can make a tomato-chile noodle soup without it, but it will not have the same Northern Thai character. Northern Thai Red Cotton Flower gives nam ngiao its rustic chewy texture and part of its earthy depth, which is why many Lanna cooks treat it as essential rather than optional.
If you absolutely have to improvise, shredded dried mushroom can echo some of the chew, but it will taste different. For the dish to feel like nam ngiao, dok ngiu or dok ngiao is the ingredient to use.