Do we even know that a Karuna has enough mass to make a successful ramming attempt against an Orion?
After all, the Sixth Wonder taught us what a significant mass disparity does.
1) The Carthage was already significantly damaged.
2) A Karuna is pretty big--not a destroyer, sure, but larger than any GTVA corvette. Couple that with the absurd number of Apocalypse antimatter torpedoes, flak munitions, and the reactor itself, and you get one hell of a punch (and bang).
3) The Karuna would be ramming at speeds around 60 m/s....that's considerable.
4) The Colossus skewered that Orion with heavy beams before it even got close. Not exactly much of a ramming attack (with poor speed, to boot).
5) Square-cubed law (IIRC) implies that the Colossus would be made out of (or built to be) far, far sturdier stuff than your average Orion. The attempted ramming was at the frontal armor of the ship, which is probably one of its thicker points.
6) I'm not sure you could even call it a ramming--I always got the impression that Koth's ship was largely gutted by beams before it really hit the Colossus (as in, it blew up before any real ramming occurred).
7) If at first you don't succeed...you've got a lot more options at your disposal. Another Karuna, and/or another Sanctus. And/or the rest of the task force just unloading into the crippled Carthage with Apocalypse spam, point-blank mass-driver shells, gauss cannon fire, and even some PDT and Burst Flak fire for the cherry on top.
Think of it this way:
Canon course of action:
1) Best case scenario: the Indus and Yangtze escape to safety, the rest of the Wargods are dead, and perhaps some barely-moderate damage is inflicted to the Impereiuse. The Carthage escapes with only moderate damage.
2) Worst case scenario: The Wargods are all killed, the Carthage escapes with only moderate damage, and the Imperieuse barely gets a scratch.
Ramming:
1) Best case scenario: the Katana (or Altan Orde) rams the Carthage, killing it and hampering effective beam cannon fire in the vicinity. The rest of the task force engages in a fighting retreat, with the Karunas reverse-afterburning while pounding away with their gauss/mass driver guns, aiming for the forward beam cannons on the other ships. Apoc torps are also spammed. The entire rest of the task force escapes.
2) Worse case scenario: The Wargods are all dead, but the Carthage is completely dead (along with her entire crew and air wing). Combined with the heavy beating the entire Carthage battlegroup took that day, and the loss of the core of the entire battle group's officers/leaders, effectively renders this group out of the fight for the remainder of the war, while time and resources must be taken in getting them out of Sol (along with repairs) and finding a replacement (and getting them up to speed/experienced). Further, the Imperiouse and Hydra are unlikely to be able to participate further in the battles of Delenda Est day.