Absolutly not, like JuleTron said, Siege Frigates are in need of a buff already. If planetary bombardment becomes harder, then Siege Frigates are gonna require a MAJOR buff.

The closest technologies in Sins that "mitigates" bombing damage is the TEC's Shield Generator (reduces planetary bombing damage by 30% unupgraded, 50% with level 1 Superior Planet Shields, and 80% with maxed Superior Planet Shields) and an Entrenchment artifact.

Upgraded planets already take forever to destroy without alot of wasted resources with siege frigates, why would you want it to take longer?

With mitigation, planets will gain a minimum amount of time that they can be expected to hold out. No matter if you send 2, 5, or 20 siege frigates, the planet will be able to survive a few volleys due to mitigation.

Currently, 10 siege frigates are 4 times as good as 5 siege frigates. Think about it. Double the siege frigates take twice as long to kill, and are doing twice the damage during the time. With mitigation, 10 siege frigates will still survive twice as long, but will have diminishing returns on damage. This means they won't be gaining exponential planet killing potential as you add more, the curve gets flattened a lot more.

With this change, siege frigates gain a LOT more flexibility in being balanced. This means siege frigates can be buffed without causing undue issues in the game. That's the main idea, to provide a comfortably sized balancing platform for a siege buff.

Capital ship abilities would have to be tweaked such that they also are affected by mitigation, or they'd race miles ahead in siege power. Percentage damage such as hysteria and drain planet would need changes so that they (drain planet) can't be used to gain undue benefits.

I dont see the need for this, perhaps you can explain? What are these siege focus raids? You mean what AI uses? Trivial to defeat at best, no player will do this because it does not work against anyone with a brain.  As is you already have ability of the starbase to prevent the planet from being destroyed, so what else do you want?

To elaborate, a siege focused raid would be one where the player attempts to do as much damage to a planet as quickly as possible. Given current mechanics, with the proper abilities a planet can be glassed in less than a minute. The main idea of mitigation would be to increase the minimum time required to kill a planet, but to keep the maximum amount of time roughly the same as before.

The main reason though, is that the damage of a siege attack increases by the square of the number of ships; more siege ships do more damage while lasting longer. In addition, planetary defenses are linear, while fleet strength grows exponentially. This means that a fleet with siege ships has a huge zone of being underpowered. Yet, since small enhancements compound extremely fast, siege ships can quickly spill over into being overpowered. By adding mitigation, the extremes of siege ships being TOO good or TOO pathetic become mitigated(pun intended), allowing for a far greater range of moderate effectiveness.

Unfortunately, to go further into detail means using a good deal of math. Basically, the idea of mitigation is to make siege ships stronger on the low end, and weaker at the high end, which is something that can seriously help their balance.