There are lots and lots of recolors/reskins of the food carts, and some of them pretend to be selling different foods, but I'm pretty sure all the ones I know of sell Maxis food and not custom food. Like I have one that supposedly sells "witch's brew" instead of coffee but it's exactly the same as coffee, same effects and animations. I can recommend a few if that's acceptable, but if you want carts that actually sell custom food with new visuals and stuff, I'm having trouble thinking of any right now. (There are plenty of buffet tables that do that, though.)
Grim Sims goth-themed food carts:
http://simscave.mustbedestroyed.org...p?topic=36404.0 and
http://simscave.mustbedestroyed.org...p?topic=36425.0 I like buying my ice cream from the Grim Reaper
but it is still just ice cream!
There's a lemonade stand here:
http://simscave.mustbedestroyed.org...p?topic=37545.0
A "pickle cart" lol
http://simscave.mustbedestroyed.org...p?topic=37010.0
This Japanese site has TONS of custom food for community lots but it's almost all buffet-based, there might be a takoyaki stand/cart in there somewhere though:
http://gyoikosakura.jpn.org/main.html