It’s definitely an oddball matchup. Pretty rare to find a Cammy player AND a Sim player in the same room.
Her j.mp is initially difficult to deal with, until you learn Sim’s other anti-airs that you don’t normally use. At a far range, it SEEMS unstoppable, but really it’s just a matter of timing. Sim can beat it clean or trade (depending on spacing+timing) with st.forward and st.roundhouse. It can beat it CLEAN if you use j.fierce at the right time, but you lose frame advantage, so it’s a trade off Sim players have to decide on.
At close range, Sim’s b+jab usually beats Cammy’s j.mp clean. Depending on the space, I also use cr.jab, which I think is a severely underutilized anti-air against characters with great jump attacks e.g. Blanka, Boxer, etc…
If she’s at a range where she can jump at you, but it won’t connect, and you’re fairly close, my favorite anti-air in the whole game is db.roundhouse ~ st.strong or st.fierce. Does a LOT of good damage AND pushes them back far away. One of those cool instances where you get to use a 2-hit combo anti-air. Very sexy to use against Cammy and Honda and sometimes Boxer and Gief.
The problem is that Cammy can beat your b.jab option CLEAN with a j.roundhouse. She can also do the same with her neutral jump to trade favorably against Sim’s standing kicks. So the reason why it’s tough is that it’s literally a good mixup whenever Cammy jumps at a variety of ranges, so you have to mix it up, stay on your toes, and make good reads on your opponent’s tendencies.
In the case of her jumping at you close range, I tend to just use b.strong early. Either it’ll hit her early before she presses an attack button, or it’ll trade, or she’ll stuff you clean up high, so you can sac throw her when she lands. Pray your timing is good, or else she might just uppercut you in the face when she lands.
Very fun matchup. One of my favorites in this game to play!