まだしも expresses 'if it were (A), that would be somewhat acceptable, but (B) is not.' It sets up a contrast where the first option is seen as relatively tolerable, though not ideal, while the second is clearly worse or unacceptable. It is often used in criticism or complaint.

This pattern appears in sentences like 'A まだしも B' where B is the actual situation and is judged negatively. It can be used with verbs in their plain past form or with nouns using なら/は. The nuance is that while A is not great, it's at least better than B.

まだしも is fairly formal and common in written Japanese, but also appears in spoken language when making a pointed comparison. It is not rude, but it can sound direct or critical, so use it when expressing dissatisfaction or a strong opinion.