This expression is used to talk about the quality of something, so you could think of it as neutral. But we usually use it when talking about something that is not sufficient, or something that is satisfactory. I don't think I have ever heard someone use this phrase is an extremely positive sense. A manager or a professor might say "Recently, your work has not been up to par." You can say to your teacher “I will make sure my essay is up to par next time." (You would say this if your work has not been good lately) (By the way, this is a phrase that comes from the game of Golf) Good luck!