I don't think that the bomber itself is the problem. Most bombers seem relatively balanced, with some notable exceptions being the Red Cat (basically a good VHF with an SN) and the Barghest (flying coffin). I believe both are getting a makeover in the next mod.
The problem is the potency of the supernova gun. I don't have access to the numbers at the moment, but I recall that the bomber SN's damage output is many times greater than a BS's equivalent weapon, the Battle Razor (or Mortar).
Obviously there is a reason for this discrepancy, probably a game-play issue. Despite the fact that the FL universe is full of NPC fighters, most players hold to the belief that too many player capships in the game makes for a unrealistic roleplaying experience. Enter the supernova, the bane of every pwnzer capship pilot. If a capship pilot (even one that can rp reasonably) gets fed this SN humble pie frequently enough, it is likely that he or she will give up on piloting his or her capship and find something smaller. This isn't a rant or even a complaint - it just how I see the bigger picture in all of this.
I have three suggestion on how to solve the bomber issue:
1.) The Starlancer bombers: Remember those? Big, ungainly torpedo bombers that could deliver a horrific payload, but absolutely needed fighter protection. What if we had these type of bombers in the game?
2.) The supernova's damage output reduced by 20-25%.
3.) The supernova made into a munitions-based weapon system. A torpedo. Give it a good speed and tracking rate. Damage output stats would remain the same as the current model. The bomber would still dish out serious damage on capships, but would have careful about using supernovas on fighters. Supernova torps could be countered by cds.