I think this sounds very playable.
First, the ice will increase AT, say +3, then2 and finally +!. This means most ships are probably just "structures, AT 21, unless you want AT's over 23. If the larger Mk weapons are used in the game, then a higher base AT, say 23, may be appropriate. My thought here is a ship tough enough to profide survivability without conferring indestructability, and the higher AT require very heavy weapons to do much damage at all.
You can handle the hits two ways: as adding tototal HP or as being shot off before damage is delivered to the frame (which could allow the ship to ignore or reduce crits till the ice is shot off, increasing survivability again). This option could allow an AT to drop "below" 21, which could be a good ship flaw (Suffering a penalty to be hit).
The above thoughts are written with the idea the Ice armor would degrade in four steps: normal/ -1 value / -2 value / -3 value. Thus maximum bonus to AT for an Ice covering is +3. I guess it could be much higher if desired.