Senpai was annoyingly perfect, like a superhuman. So that one 'quirk' was actually her cute side... or more like, her charm point.
Literal
Senpai [topic-は], sarcastic-[to-the-degree-of-なくらいの] perfect-superhuman was-[polite-past-でした][reason-から][confirmation-ね]. So conversely that 'quirk' [topic-は] senpai [attributive-の] cuteness... [or-rather-というか], charm-point was-[polite-past-でした][assertive-よ].
Character-profile style typical of anime/manga dialogue. 先輩 is the senior-figure term that captures a social role English has no exact match for. 嫌味なくらい 'to the degree of being obnoxious' is a common intensifier pattern — ~なくらい attached to an adjective means 'to the point of being X.' 完璧超人 ('perfect superhuman') is a stock hyperbolic label. 可愛げ ('cute charm') is an abstract-quality nominalization of 可愛い + げ 'appearance/quality of.' The conversational corrector というか ('or should I say, or rather') marks a mid-thought revision. チャームポイント is wasei-eigo for 'endearing trait / best feature.'