Alun Davies, who is a Labour AM for Blaenau Gwent, said John McDonnell was "out of his depth".
He made the comment after Mr McDonnell produced Mao Zedong's Little Red Book in the House of Commons, saying Chancellor George Osborne was selling off Â£5b worth of assets, in particular to China.
Mr McDonnell said the reference was "a bit of a joke".
He quoted Chairman Mao to Mr Osborne and accused him of "sheer economic illiteracy" in a Spending Review speech on Wednesday.
But Mr Davies tweeted: "McDonnell is a clown. Way out of his depth. We needed leadership, strength and substance today. We got Mao".
The shadow chancellor has been branded "a clown" by a Welsh party colleague.