Mr Cameron used the word during a furious exchange at Prime Minister's Questions over the part-privatisation of the Royal Mail, during which Mr Miliband branded him "the dunce of Downing Street" and accused him of selling off the company at "mates' rates" to his friends in the City of London.
"Muppets" is not banned as unparliamentary by the Commons rulebook, Erskine May, and Speaker John Bercow made no comment on its use by the Prime Minister, but its deployment in the chamber raised the eyebrows of some at Westminster.
Appearing on television, Mr Cameron insisted the word was "not inappropriate".
"These are spirited exchanges," he said. "I think the language in the House of Commons, where there is a lot of banter ... I don't think that's inappropriate."
Mr Miliband challenged the Prime Minister to explain why part of the Royal Mail had been sold at a price £1.4 billion lower than its current valuation.
Mr Cameron said: "The case I was making was that Britain has benefited from the sale of Royal Mail to the tune of £2 billion - money that Labour never would have achieved because they never managed to privatise that business."
He said he was not willing to accept accusations over the sale from "the people who advised Gordon Brown about selling Britain's gold and losing £9 billion".