Simmons is very good player (perennial all-star) who will likely improve. Of all the skills NBA players have successfully developed, I'd imagine shooting is the most common.
A lot of otherwise really good players significantly improved their shooting -- Rondo, Kidd, Paul, Horford, LeBron, etc.
If you look at Simmons' earlier highlights, his form and makes from a variety of scenarios looked much better. These guys play basketball all day, year around; I would think poor in-game shooting is more related to confidence and mindset rather than ability, especially surrounded by so many phenomenal snipers.