"thick as mince" means a scottish simile meaning extremely stupid — 'thick' being the british slang for dim-witted, and 'mince' (ground meat) representing something lacking coherent form or substance. It is one of many casual words teens pick up from friends, social media, music or sport, and on its own it carries no particular warning. If you hear your teen use it, it is fair to ask what they mean by it in their friend group, since meanings drift quickly. Asking out of curiosity rather than alarm tends to keep the conversation open and useful.