no oil painting

be no oil painting {BrE, humorous} (to not be attractive) — не красавец, не красавица; не писаный красавец; не ахти какой красавец; не бог весть какой красавец; смотреть не на что; ...
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Example 1: She has an interesting face but she's no oil painting. (CID)

Example 2: She's no oil painting but she's got a lovely personality.

Example 3: He made fun of her looks, when he's no oil painting himself.

Related Vocabulary:
be not much to look at {infml} (to not be attractive)

Example: The house isn't much to look at but it's spacious.

mutton dressed as lamb {BrE, infml diapprving humorous}(a way of describing an older woman who is dressed in a style that is more suitable for a younger woman)

Example: Do you think this dress is too young-looking for me? I don't want to look like mutton dressed as lamb.

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Related vocabulary:
[ugly as sin]
[ни рожи ни кожи]