It was banks that were on the wrong planet, with accounts that vastly overvalued assets.

这是一个强调句。on the wrong planet：习语。“脱离实际；与事实不符”。

with accounts that vastly overvalued assets，是状语不错，是原因状语。修饰整个谓语部分： were on the wrong planet。相当于说：because they had accounts that vastly overvalued assets.

句子可演绎为：

These banks were on the wrong planet, becuase they had accounts that vastly overvalued assets.

【翻译】这些银行脱离实际，（因为）它们的账目上充斥着估值过高的资产。