Up tilt: an awesome fake out, does 12%, has invincible legs, but has really bad endlag. Great to use out of a spot dodge. much harder to do without a Cstick set to attack (at least, it requires a lot more getting use to). Ftilt is a great defensive poke. While not safe on shield, it is still a good tool in footsies, and should be very accessible. Dtilt is kinda meh. Does more damage than Ftilt and has slightly more range, but has really bad endlag. I usually would just use Ftilt. Using the Cstick for attack allows you to use aerials freely without changing your momentum to the direction of the aerial, allowing you to perform retreating Forward airs, advancing Back airs, et cetera. Considering ZSS's incredible aerial gameplay, this is a much better set up. ZSS's smashes are really not that good anyway aside from Dsmash. ZSS will mainly kill with up B, Down B, Up air, and Back air. Setting the Cstick to attack also allows the use of "double sticking" a method of aerials that can be seen here:
https://www.youtube.com/watch?v=BHnTF4GQQDA
Setting Cstick to tilts also allows you to use tap jump (jumping by pressing up on the joystcick, as opposed to with a jump button) freely without worrying about being able to use your tilts effectively. many people with c stick set to smashes is that they have a hard time performing tilts, especially up tilts, as opposed to smashes. This is due to the necessity of having to lightly tap the directional pad and press a to get tilts and having to hit it harder to get smashes. sensitivity of the button making different moves come out is much harder than just having two ways of performing two different kinds of moves. (Cstick for tilts, direction +A for smashes.) I have never heard of anyone having trouble performing smashes as opposed to tilts. Overall it is a much better setup for ZSS, and in my opinion a much better set up for pretty much any character, as it allows much better control of your character's movements and allows for more combos.
And as for ness's tilts, Utilt combos into itself and into Uair very easily, so it should have some use. Dtilt is basically lagless, can hit multiple times kinda like lucas's , and has good tripping ability, allowing a combo into Fsmash. it is a very good kill setu similar to diddy's banana into Fsmash. banana is basically an auto tripping projectile. Ftilt, idk . i dont think it is that good. Anyway, I hope this helped.
Here is a video to help explain the Cstick more:
https://www.youtube.com/watch?v=YKXJDXgcZps