Ann Widdecombe branded "stupid" over his.
The Foreign Secretary has faced pressure in recent days for past remarks about the US president-elect including calling him a "neo-Nazi sympathising sociopath".
Former Tory minister Ms Widdecombe told GB News: "I think first of all, Lammy is stupid anyway, quite apart from the things he said about Trump.
"Certainly having a Foreign Secretary in place who is capable of coming up with those sorts of judgments is very, very dangerous. I mean, you need diplomacy, you don't need abuse.
"So I don't think he's a good appointment anyway, but come on, you can't guarantee that anybody's still going to be there for four years. I mean, no Prime Minister can guarantee that; you don't know what's going to happen.
"He's a lawyer, you would think he was a bit sharper than that."
Ms Widdecombe suggested Reform leader Nigel Farage could act as a bridge between the Labour Government and Mr Trump, who he is friends with.
But the former Brexit MEP said it should be an unofficial role rather than being the UK's ambassador to the US.
Ms Widdecombe said: "I think Starmer could use him very usefully behind the scenes.
"I don't think he should have a formal role like ambassador. He's got his role, he's the leader of Reform, he's an MP for Clacton. That is what I think his main role should be.
"But because he does have an excellent relationship with Donald Trump, Starmer would be a fool not to tap into that. But I don't think you'll find our Keir is going to be terribly keen."
Sir Keir Starmer was left scrambling to build bridges with Mr Trump after his stunning US election victory.
Mr Lammy is one of a series of senior Labour politicians who have been critical of the Republicans in the past.
The Foreign Secretary yesterday dismissed his previous comments are "old news" in an interview with the BBC's Newscast.