No one likes to see a scratch on their mountain bike, but the sport sort of means taking risks. Over time bikes will get dings, scratches and nicks in the paint. Some consider this a rite of passage, a way to see that the bike was well used. A bike with lots of nicks could be considered a bike that someone really loved to ride.
But the All Mountain Style Frame Guard can help reduce the nicks on the downtube. It is made of semi-rigid PVC that is flexible enough to adhere to most frames, and can protect the frame. It applies easily, and can be removed with a heat gun or hair dryer. It is even available in clear so you won’t notice it but will know it is protecting the frame. Of course you could just consider the nicks to be character.