I can hit gunboats with a light mortar in my carrier, i'm sure you can hit them with a razor in your much smaller and more manuverable ship.
It will take longer to kill another cruiser with only razors, you can still go with a single LM if you want, but each shot with a mortar is going to eat up about 2/3rds of your energy. In the long run, a mortar will do more damage-per-energy than a razor, so it might be a better choice, you're just going to have to deal with waiting for regenerations.
If you go with a mortar, make sure that your shots count - dont miss or it's going to hurt you more than a missed razor would. Even on my carrier with 3-4 times more energy than your cruiser, if I miss a mortar shot, it can turn the battle against me when fighting other caps. I'd say get lots of practice if you're going to use the mortar.
Still, even if you stick with a mortar, get two pulse cannons - they're far more efficient against enemy shields, very easy to hit with, and have a fairly high refire rate.