、。
I cook, but I'm ridiculously bad at it.
Literal
I [topic-は] cooking [object-を] do [but-が], absurdly bad [copula-です].
下手くそ is an emphatic form of 下手 ('bad at, unskilled') — adding くそ (literally 'shit,' used as an intensifier for 'terrible, very') for colloquial strength. とんでもなく ('ridiculously, absurdly,' adverbial form of とんでもない 'absurd, outrageous') ramps up the self-deprecation. The whole sentence is self-mockery: 'I do cook, but not well.'