Tout de même ! is a set expression that marks an opposition, similar to but, however or nevertheless.

The sentence can be translated to:

Okay, okay, but how annoying you can be!

The other answer is correct !

I’d add that Le petit Nicolas is a book in which the author took pleasure in transcripting all the “language faults, simplification and nonsense” children add in their sentences when they talk.
He did that to emphasize their childish behaviour and give his books a very unique feeling. As if you’re in the child thoughts.

So careful about improving your french with these books. Some expressions/words are not used irl the way they are in the book.