The best is to have a "balenced player", I don't say that he must be level 11 in all the caracteristics but with LoD's example we can clearly see the most common problem for a new manager.
A player must be active in defense and in offense, you can always have some difference for the level but search a little on the transfert market and you will see a lot of player with level 18+ in Jump Shot and nothing in defense, handling, ... This kind of player have a salary around 50K/week.
On a other hand you can find some guys with 13-14 in JS, 12 in Defense, handling, etc. with the same salary and this player is really helpfull.
Why ? because the first player will take 254687 shots/match because it's the only thing that he can do and because he's clearly the best shooters of his team but he'll take also a lot of bad shoots with a defender against him, etc.
The second will take somes shoot but with his good handling, jump range, dribbling, etc. He will take his shoots in better position and his % will be higher.
So to answer to your question: YES an higher salary means that the player is NORMALY better but only if his caracteristics makes that he will play right and good.