There are ways to move out of arc and prevent arc from being gotten on you.
If you are in arc, yes greased mortar will just utterly destroy you.
Again:
- What were you engineers doing? Why is your armor down. The Galleon has a TON of armor. This is why it's a tank.
- What were your teammates doing? Why haven't they distracted or broken up the focus fire?
- Where were you in combat? Why did you not take evasive action?
There are a lot of things at work here. Under very specific circumstances, yes a lot of things will seem OP.
They are not my crew, I inform my crew every time that when hull armor is down, we are going to die instantly. Why? Because everyone and their mother has a mortar on his ship; it's firing pattern is even more consistent than the flak. I've seen other people's crew's ships die at a blink of an eye just because the hull armor went down and I started firing rounds. It is just too easy. There isn't even a guessing game involved, hull armor down -> ship dead, which unlike the flak is guaranteed.
Furthermore, it doesn't matter whether they were repairing or not, that chaingun will eventually take down the armor and it will take them a considerable amount of time to rebuild it. Now, when considering lesmok, it would be easy for them to fix the armor in time and let the armor take most of the explosive damage. But I seriously doubt that anyone fends off a greased burst; no matter how hard they try. That is head on damage.
You recommend evading the damage, and I'm pretty sure everyone here would love too. But short of moonshining out of range(and breaking my most likely damage engines) I don't see any ways of avoiding the firing arc of a mortar. Maybe Hydrogen could work. But regardless, it's still unlikely that a ship like pyramidion can be shaken by a Galleon. The Galleon in all its glory can fight, put up a manly fight, but evasion isn't exactly its thing.