As John Lawler points out, several amusing combinations of negations exist, although your qualification “mundane on the outside but with a heart of gold” limits possibilities somewhat. Here are a few words and phrases to consider.
• unpretentious, “simple, humble, not pretentious, plain”
• down-to-earth, “practical; realistic; pragmatic”
• salt of the earth, “A most worthy person ... A decent, dependable, unpretentious person”
• solid, “Large, massive ... Strong or unyielding ... Excellent, of high quality”
• earnest, “Serious in speech or action; eager; urgent; importunate; pressing; instant. ...
Ardent in the pursuit of an object; eager to obtain or do; zealous with sincerity; with hearty endeavour; heartfelt; fervent; hearty ... Possessing or characterised by seriousness; strongly bent; intent ... Strenuous; diligent”