Attack runs on capital ships are always fun! You got the basic ingredients for this sort of encounter. Impressive too that you did everything here by yourself.
There's a lot of things you could do to improve it, but I'll suggest these 3 which I think are pretty important:
1) Feedback. The player ship should flash or have some sort of explosion when hit. It's too had to tell damage when the only indicator is just the small HP counter in the upper left of the screen. The turrets should also do something to indicate if you scored a hit on them or not.
2) Textures. I didn't realize that the guns had barrels until I was very close because everything on the mothership is the same texture.
3) Variation. Increasing the difficulty of the turrets is fine, but I think most players would be more interested in fighting different types or quantities of ships.
Hope this helps and I'm looking forward to future versions!