I think that June or July are the best months as you have almost permanent daylight. The weather tends to be mixed (it is rather like the north of Scotland) but the beauty of the scenery compensates for the odd drop of rain. You are spoiled for choice as far as cruise lines are concerned. I went on one of Fred. Olsen's ships and was delighted with the whole cruise. I am sure that SAGA, P&O etc are just as good. You should find it a worthwhile trip. Happy cruising.
Check out the weather section on the BBC site. There is a year round weather summary for Bergen which shows even the best months have lots of rain!. June and July look OK but when I am going there in September there is a good chance I won't see a single Fjord.