I've read that Streep's portrayal is spot on as far as mannerisms, voice, style, appearance, etc, but that the writing and direction makes Thatcher out to be an eccentric, stupid nutjob. The Reagan caricature applied to Thatcher.
No doubt Streep is a great actress if one is into those hoity-toity dramas and period pieces where everybody speaks with a fake British accent. Not my usual cup o' tea.