If you are a private pilot doing low hours, remember that cotton burns like a torch, while wool and silk merely frizzle up and extinguish. So if there's a choice, wear silk or wool in preference to cotton.
If you really like wearing cotton, there is a cotton product called Proban. Hitherto, it has been possible to obtain flying wear made in Proban, but I am not sure about its current availability.
I believe there are also simple, water-based processes you can apply to an existing cotton flight suit that will reduce its flamability. Maybe the cotton trade associations can advise on these.
There is a product called MSI Firecheck that can apparently make cotton fire-resistent, but I know almost nothing about it and don't know whether it is even suitable for use on clothing. There should be details somewhere on the Internet.
Cotton might burn, though I I would question "like a torch",at least it does not burn into the skin like nylon. MSI Firecheck is used in the film industry and is supposed to be very effective, but washes out so needs reapplying if you wash your kit!