does fatter yoyo string help with the slack/ whip tricks? or not?
You know, people say it does. But I honestly don’t find that thicker string whips or slacks any better than regular thickness.
People seem to swear by thick string though.
I agree with Glenacius, I’m not sold on fat string helping with slack. That said, my favorite string is YYSL Type X. I also like the YYE Core. Venom and Ammo are nice, but, sometimes they feel too thick (probably due to sloppy technique on my part)
I use YYSL Ammo exclusively, but for me it’s just a comfort thing. As mentioned, I didn’t really notice much of an increase in slack performance over a thinner string, it’s just more comfortable on my fingers (especially the one with the slipknot on).
I remember when I first used Ammo it seemed so thick, but I’ve been using it for so long now it doesn’t feel thick at all. I bet regular string would feel like dental floss to me now. ;D
If you want string that slacks and especially whips better than regular, you’re looking for slick string like Toxic (Original/Dragon/BG1), or something like YYSL Venom (not as slick as Toxic, but slicker than regular YYSL string). 
I feel like it does because of more string weight. Fat Kitty for example, is made using more threads, so I do find it whips better.
I did recently make a blog post about Fat kitty, so if you are debating buying it, this might help