here "菜" does not mean a dish or vegetables or greens
when i am not good at doing some thing or i am elementary/primary,i may say "我很菜",and why i say these words?
because i want to get help from some one who is skillful in that domain. at the case of some one said "你很菜",that may mean you can do better,and sometimes just kidding words .but it also have some unfrendly way of use,depends on the language situation.
and i wish the words above may helpful to you!