Its perfectly fine for Ingame clearing as salamence on it's own is a powerhouse and good nature's arent necessary for the main story.
The Battle Tree is a different animal. These Pokemon and movesets are much better and have some competitive investments in them. Its completely possible and you still may succeed but that 10% lost to speed you dont have from nature's may make you lose alot of speed ties and the 10% less attack may make alot of normally OHKOs 2HKOs. However you also may live some otherwise lethal physical hits with a sliver of health.
Competitive Battling is where your weakness will most be shown. Most people use just ideal setups and your lack of stats where they really matter will lead to a lack of production more often then not from Salamence. It still can be effective but vs an Ideal Salamence you'll fall flat almost every time. I recommend Jolly, Timid, (depending on your preferred set) or Naive(Mix Mence is Best Mence Imo).
Hope I helped.