Her cooking skills are not great; she can only make simple dishes.
Tips
usage
不善 has two distinct uses: (1) 不善于 / 不善 + verb: 'not good at something' (e.g., 不善于交际 — not good at socializing); (2) standalone or in 来者不善 — meaning 'ill-intentioned' or 'not to be taken lightly.' Context determines the meaning.