I’ve got some Humvees which are ideal for the 5 mile commute in winter. Not sure I’d use them for MTB unless it was absolutely perishing though – they can make my legs pretty warm (which I realise is the idea, but they might be a bit much :-) )
Edit: In answer to your question, they’re always going to look like cycling trousers. The velcro cuff can be loosened off so they’re not tight round the ankle, but they’re definitely still tapered in appearance!