So, I have already added this into the code(in the dev version). Here is the logic.
You have 4 types of damage 4 types of armor
Before I used to adjust them as I felt but I found this to be both difficult to balance and frustrating for new users as some explosives were less effective than others. While this will still be true.. the bonus to damage has been made much easier to follow.
The above image shows the multipliers to damage. So for example an explosive weapon will do 0.5 times it's damage to a heavy unit and 2 times it's damage to a building. Some of the logic may seem odd so here is a detailed explanation.
Why does explosive deal so little damage to heavy armor?
Explosive weapons explode on heavy armor doing limited damage, where as a beam weapon will melt right through.
Why does light armor take 2 times as much from light weapons?
The light weapons were designed to penetrate low armor targets. In this area they are highly effective. Light weapons are things like machine guns like 30 and 40mm weapons.
So does this mean that we will have hard counters?
Yes, because some things are just better at destroying others. No, because you can still kill gundam with small arms if you hit it with enough. Which makes sense to me as some of those small weapons may find their way to vulnerabilities in the ms system. So it isn't cut and dry, just easier to understand what to use against what.