Requiring insurance to own a gun might not be a terrible idea for dealing with America's gun problem... with costs scaling like cars - lower premiums for less powerful/lower capacity models, especially if kept in a secure gun cabinet. Higher premiums for types of gun more likely to be involved in crime or accidents.
Mandatory Insurance for small model aircraft?... well, there's a risk of injury or property damage, but is it really any higher than the risk from a bicycle or skateboard? - All could kill in the case of a very unlucky accident, but the chances of more than minor injuries or property damage is very low.
A lot of model aircraft enthusiasts (via their clubs) already have some level of insurance, at least when flying at club fields. Traditional model aircraft - fairly large planes and helis - have been rather more dangerous than the small electric models that are popular these days, so safety has been taken very seriously by most people involved in the hobby.