Sim’s AA game is anything but trash, in fact I would argue it’s one of the best in the game. He has an AA for every angle, and importantly, he has an AA for right over his head, which is really important; not every character has that. He also has a pre-emptive AA if you’re on point that can CC, so there’s that. Here’s a run down in my opinion of his AAs (after using solely Sim for the last while), and the ranges they are best used at, followups etc. If anyone disagrees or has anything they’d like to add, please share! 
st.jab - 4f, super quick, godly hitbox. This is your go to AA for when people are just above your head, or jumping over you. This thing beats everything. Really helpful in the corner when people go for NJ to bait throw, as you can still recover in time from a whiff throw to st jab em. After a st jab aa in corner, I like to lk slide under my opponent to corner them and get some meaty pressure going. Even midscreen, if you st jab aa, immediate lk slide will switch positions, and is hard to block. You get a free meaty cr hp/gale/throw after the slide, so it’s a godly AA + a pressure starter. St jab AA is a must when playing Sim, as it AAs a lot of moves that other normals won’t (Nash moonsault, Dictator’s devil reverse etc)
cr.MP - Really useful AA at a tonne of ranges. This normal is really good at most ranges which aren’t directly over Sim’s head. It’s my go to AA when midscreen/neutral and when st.jab isn’t needed/wont reach. cr.MP also nets you some pressure, as you can cancel the cr.mp into lp fireball for some slide pressure and ground control. Be careful not to cancel to fb when you AA with this when your opponent is close, as they will land close enough to punish the fireball animation. Only cancel to fireball when you AA at a good distance; your opponent will try to cover said distance upon landing and will run into the mk slide fireball combo =). I found this move trades at times, but even still, that’s fine, since you recover first and can go pack to poking them in the neutral, or go for an IAT, or whatever.
b.MP - One of Sim’s weaker AAs, IMO. Like cr.MP, it’s useful when your opponent jumps at an angle. I use this one the least honestly, everything this move does, cr.MP can do, and more. B.MP is useful for when your opponent is close, and in scenarios where cr.MP might whiff. Even then, I just use st.jab at those ranges, since b.MP seems to trade quite a bit, which sucks because on trade the opponent still lands in front of Sim, which isn’t exactly a good position for Sim (you took a trade, and they got in). Honestly I feel like cr.MP is way better than b.MP, I rarely ever use this normal if at all. The angles where back mp is good, are few, and even then st jab seems to be better (sure, it does less damage, but you risk them being in your face on trade, and you get lk slide crossunder etc after st jab, as opposed to b.MP). Maybe it does more damage, but honestly it feels like that’s not enough to warrant using this over cr.MP/st.jab
b.HP - A very situational anti air, as well as being very slow. You really need to make a read on a jump to hit with this move, I find. Still, it has its uses, and it’s not a bad idea to pre emptively throw this move out when you think your opponent might jump. Even in the neutral when you are at a safe range from your opponent, doing b.HP isn’t a bad idea since it will punish jump happy players and will likely go unpunished in the neutral. It’s not a super slow move recovery wise, so I feel like you can throw it out with impunity most of the time if you think someone will jump. It also tends to CC quite a bit, which in itself leads into a mixup when you cancel to ex fire. This is a useful AA that is worth incorporating into your game.