Worth noting that the Outer Space Treaty prohibits “weapons of mass destruction” in space, though that is generally interpreted to apply only to nuclear weapons.
An asteroid (or even just a huge - say 100K tons - space transport barge full of ore) with a huge rocket engine attached - until it is targeted as a weapon, it is hardly can be considered a weapon (yet for example 2000 ton coming in at 10km/s would be a Hiroshima TNT equivalent). And for MAD you do need to have credible threat to wipe your opponents Mars/Moon/etc. colonies too.
"Starship Troopers" (the film) had an asteroid as a city-buster.
If there's anything we know about treaties it's that everyone always follows them
MAD in an ASW context means 'magnetic anomaly detection' not 'mutual assured destruction'.