Prime time for the mall is Saturday afternoon on the weekends (say around 2 p.m.) So avoid it if you can. Right when the stores open or a couple hours before they close on Saturday is a lot more chill. Sundays are a little quieter, but most stores have shorter hours too, so your "shopper-tunity" is considerably less. Another good option is Friday night because most of the weekend sales start on Friday but most shoppers are doing other things in the evening.
If you have a more flexible schedule during the week, then weekday mornings at the mall are ideal for a quick in-and-out. If you get there right when the stores open, you won't encounter many more folks than employees and a few mall-walkers. Lunchtime tends to get a little busier but traffic tapers off again in the afternoon hours. Independent boutiques are always more low-key than big department stores any time of day. If you have an all-day shopping trip planned, it might be best to start at the mall, have lunch, and then visit the boutiques in the afternoon.
Mind you, all of this goes right out the window during the holidays. If you're not OK in a crowd, then shopping is pretty much a miserable experience from open to close between Thanksgiving and Christmas – unless you take advantage of those extended hours and show up at 9 or 10 pm. The good news is that there's never a bad time to shop online.