Ok, few thoughts from a newbie.

People at these levels generally suck. But don't take my word for it - or anyone elses for that matter. You need to witness it yourself and here's how you do it. Play one table only. Open up a hand history window next to the window. Every time a hand goes to showdown read the hand history and every time each of the people involved in the hand act you consider if the action they made were correct or not based on what they actually had. If you see them doing something that you think is probably notably wrong, take a note on that player. Doesn't bet overpairs, bets AA weakly, plays any two cards at random.. the possible comments are endless. Just take them down on the player.

You will be taking the same notes on lots and lots of different players. This is good, as it will make you not just conclude that they are terrible, but also exactly in which ways they are terrible. You'll be able to conclude some general trends about 'players at these levels' and can devise strategies that effectively counter them.

As you get better at the whole thing you'll want to get away from sweeping generalizations about players at a certain level or with certain stats, but they're a good stepping stone to learning to predict what your opponents are holding and playing.

Don't assume that 80% of players know what they're doing. Also don't assume that 80% of players don't know what they're doing. Instead - take down the facts of hands you've seen them play and use THAT rather than arbitrary assumptions.