I'll try again...
Going by the 普遍的な傾向 definition, I can understand why ものだ would be used in the sentence こんな難しい仕事を新人に任せるなんて、むちゃというものだ. I.e. it's generally accepted that it's irresponsible giving difficult tasks to newbies. However, I still can't wrap my head around 人の影口を言うものではない because I can't see how もの functions here. Obviously you can tell me what it means but I'd end up with the same problem as the OP in that I would never know how to make my own sentences using it. If it was me I'd say something like 人の影口は言うものではない.
(btw, 人の影口を言う mean's talking behind someone's back, right??)