。
She isn't so much slim as actually skinny.
Literal
She [topic-は] slender is rather-than rather thin is.
The sentence draws a careful distinction between two ways of being thin. すらっとしている ('slim, slender') is flattering — it pictures someone svelte and well-proportioned. やせている ('thin, skinny') is neutral-to-negative — it can imply being underweight, gaunt, or not robust. The pattern AというよりもむしろB ('not so much A, but rather B') replaces one description with a more accurate one; むしろ ('rather') reinforces the swap. Japanese has a rich vocabulary for body types because the distinctions matter socially — different words flatter or critique.