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They don't seem like it on the surface, but they're actually mean-spirited.
Literal
Appearance [topic-は] so is-not [but-のに], truly [topic-は] disposition [subject-が] bad.
A character observation. 見かけ ('outward appearance,' 'how something looks') pairs with 本当は ('actually, in truth') to set up a classic contrast between surface and reality. 意地が悪い is a fixed expression: 意地 ('will, disposition, spite') + が + 悪い ('bad') = 'mean-spirited,' 'spiteful,' 'petty.' Literal sense captures the idea of 'having a bad disposition.' Common collocation for describing nasty characters, often with an implied edge that the behavior isn't accidental but rooted in personality. The ~のに conjunction ('despite/even though') sets up the contrastive structure.