Ah sorry, misunderstood the question

Yes, assuming your resistances are working as they should be then Magic Resistance covers all the elements (Fire, Frost, Shock) assuming those elements are generated by spells (presumably including dragon breath). If you stand in a pool of oil and torch it then Magic Resistance won't help you vs Fire though.
You might wonder why bother using Elemental Resistance at all if Magic Resistance covers elemental damage (when generated by spells), given you have no plans to either set yourself alight or prance naked through the snow, and generally speaking Magic Resistance is the superior form of protection. Except (as you say) Magic Resistance caps at 85% whereas Elemental Resistance stacks up to 100%, and it is always possible you might get fried by a mechanical trap...?