If you are going for Critical hits, none of the stats aside from Senses and Critical Hit matter much ... that said ...No matter what type of physical combat you are doing the more times you attack per round, the better off you are (more damage and more chances - each swing - to get a Critical). Both Dex and Spd affect this (one affects how many attacks you get per round, the other affects how many swings per attack you get .. right click on them to find which is which).
Strength really matters based on the weapon type ... I think "bash" type weapons factor in strength into damage. Again, right click on both strength and the weapon of choice to find out for sure.
And of course to be in the round fighting, you kind of still need to be alive, so your ninja will need hit points ... and being a fairy won't get that many, so you may need to pump up vitality.
No, they don't make it easy to play a FN for the CoC 
Tools for Wizardry(r) 7