The food wasn't very good, but in every other respect, the party was a success.
Literal
Cooking [topic-は] not-very was-not-delicious [but-が], other [possessive-の] points [in-では], that party [topic-は] success was.
Two topic-は markers set up a contrast: 料理は introduces what fell short, then そのパーティーは shifts to the overall verdict. The conjunction が here is concessive ('but'), not the subject marker. 余り + negative (おいしくなかった) softens the criticism to 'not very' rather than a flat 'not at all.'