I think we can just say that hyperspace ramming is possible, but it's not an efficient use of resources. It's the same as saying that kamikaze plane attacks in general are possible however they're a really poor use of resources, whenever you have the choice to do something else, you do the something else.
Kamikaze plane attacks are technically good use of resources if they all succeded, after all a plane is much cheaper than a carrier, but the problem is that the planes were often shoot down before hitting the ships and the damage often wasn't critical.
Neither of those problems applies here, especially since you don't even have to build a whole ship to ram someone, just make dedicated hyperspace missiles or torpedoes. After all, that is literally what we did in reality - missiles are now the primary weapon of choice for many tasks such as naval combat, long range bombardment (nukes) and anti-air combat. Missiles are literally unmanned ramming vehicles that go boom after hitting something.
Basically we can just assume that the amount of damage done with a big plasma weapon over it's expected lifetime in a battle exceeds the damage done by firing hyperspace-engine things through the target, in terms of resource units needed to create the weapons. Basically, if you took the same amount of resources needed to create that rebel flagship, but instead used them to make a single huge plasma cannon (that doesn't have hyperspace engines), it would by necessity have done more damage than hyperspace ramming the enemy with the ship.
Probably yes, the damage would be greater, but the damage will be split over time during which the enemy is also shooting back. This is really simple, you're trading the ability for sustained firepower for the ability to deal a lot of damage instantly, especially since the hyperspace ramming seems to work against shielded opponents. You don't even have to kill the enemy ship outright, all it takes is for the hyperspace torpedo to cripple the enemy ship battle capabilities and then finish it off with regular weapons.
It's actually pretty easy to explain why hyperspace ramming is a thing you can do, but nobody normally does it.
No, it's actually really hard to explain why hyperspace ramming is a thing you can do, but nobody does it, since as shown by the example it has great capabilites and even if it's not a staple of space combat due to w/e reasons, it's very hard to justify that noone has weapons capable of it.