The word "ablative" is typically used in reference to reentry. As the vehicle encounters incredible heat as it comes back into the atmosphere (due to compression heating or whatever), the shield intentionally has some part of its surface gradually boil off.
Let me get what you're saying straight. Superpower A would launch an ICBM, and then Superpower B would then laser it as it flies through the air or space. As the missile absorbs the laser heat, its surface starts to boil off, but if the laser doesn't boil off enough, then the laser counter-attack on the missile could still be survivable.
Edit: I didn't think of space but in space this could even be more viable since the ejecta from the ablative heat shield will just stay with a body in free fall and further scatter incoming radiation at least a little.
I rather thought about shorter engagements where a missile only takes tens of seconds and needs to be destroyed quickly. Each second an ablative shield buys improves the chances of a hit.
Also used when nominative, dative, accusative, genitive and vocative heat shields just won’t do the trick.
</armchair-general>
But more importantly I'm not sure how you can have one and still have a missile guidance system on board, and also they tend to be pretty heavy.
AVCOAT 5026-39 which apparently is an ablative heat-shield material for NASA's Orion (and has been developed during the Apollo years) has a density of 0.51g/cm³ so about half the density of water and one fifth that of aluminum (of which a guided missile is made mostly, I believe).
From reading some reports of the performance of Apollo era heat shields it's clear though that reentry is a way more gentle affair in terms of thermal load, than a 50kW laser in a 1cm2 spot.
The endured peak heat flux per area is about 0,48 kW/cm2 for the heat shield, magnitudes off, if the 50kW laser can achieve a 1cm2 spot.
The total amount of heat flux the ablator was required to endure results in a total amount of heat energy/area of 0,0141 kWh/cm2 - which 50kW in a 1cm2 spot would saturate in about 1 second. Reentry lasted 800-1000 seconds.
What is probably way easier though (and actually survives launch and particles hitting it and is already proven in practice) is to use ablative heat shields on missiles. The materials available are already optimized for mass vs. effectiveness due to the engineering constraints in space flight. Missile optics would probably need to be coated though or distributed.
[1] https://www.osapublishing.org/ol/abstract.cfm?uri=ol-23-20-1... [2] http://ab-initio.mit.edu/book/