No. Not at all! If you have a flat tyre, and you use tyre sealant, it won’t ruin your tyre. It’s a myth that this happens. The reason people think this is because when you’re sealing your tyre with a liquid or gel, it can look like something has been spilled in it. This is just an illusion, though: the sealant is being applied directly to the puncture hole in your tyre, which makes it look like there’s something wrong with that part of the tire. But in reality, there isn’t anything wrong with the tire—there’s just something wrong with its inner tube!
Tire sealants are used to repair punctures in rubber tyres by filling them up with liquid or gel so that they don’t leak out any more air from inside the tube itself. They’re also great for preventing further punctures from happening again as well as fixing existing ones quickly without having to take apart your car and replace everything inside each one individually on its own (which would be super expensive).