There's a difference between rushdown and good up close
Rushdown characters in IGAU were characters like catwoman, Wonder Woman, flash, etc. They had all the tools to rushdown whether it be fast normals/strings, frames traps, etc
Killer Frost was just really good upclose. She had the parry, the mixups, the damage, the slide, etc but she couldn't really rushdown. Outside of her S1 or S2, her strings were too slow. She also had literally no footsies besides b1 or slide. She had really good frame traps but that all complimented her mixup game and not really helped her rush. Once she landed a hit, her vortex game started but that's when it became straight up momentum, not really rushing
I thought of her as a defensive character. She had every single tool she needed to play defense. Very fast high damaging projectile to help zoning, tracking spike to compliment the zoning, a parry to help against the rushers, top 5 d2 in the game to help against the jumpers, MBf3 was gdlk armor, etc. There wasn't really a reason to rush with her when she had all that. Everyone knew what happened once she touched you so just sit back, build the meter and let them come to you.
I remember when some frost players, or even other players, used to tell me they didn't like my play style with her because she was all about the mixups yet I played so lame with her, yet i was still placing tho so clearly I was doing something right lol. The aggressive frost didn't place at all IIRC
