The Suncrusher was designed primarily around one particular and unique purpose. To destroy a sun, acting as a weapon of mass destruction. It's foremost main and primary purpose is not built around engaging war fleets despite the fact that it can. All those point defense systems are designed as SECONDARY designs meant to throw in that additional purpose of defense. The MAIN design is not directly centralized around engaging fleets. If the designers made the suncrusher and decided its foremost main purpose was hostile fleet engagement, rather then acting as a WMD platform then it actually would have been an acceptable answer.
Second problem is that the suncrusher is actually no bigger then a fighter craft. And another problem was the logistics of said superweapon. It only carries a mere 11 resonance torpedoes in its hull rather then stocking up on hordes of projectiles as should be expected with other warships.