。
She tends to badmouth other people.
Literal
She [topic-は] people [possessive-の] bad-mouth [object-を] say tendency [subject-が] exists.
Two set expressions combine. 悪口を言う (わるくちをいう) is the standard phrase for speaking ill of someone or malicious gossip. ~傾向がある frames this as a habitual pattern rather than a one-off incident — 傾向 (けいこう, 'tendency, trend') is slightly formal, and in casual speech one might say ~くせがある or ~がちだ instead.