So in toy model 0 we assume a very generic nucleus where there is no overlap in the electron density. However to introduce overlap in this physical example it lowers the coulomb interaction between the position charges and thus lowers the general energy of the toy model A as compared to toy model 0. This can also be seen in the fact the actually addition to the energy given by the first order monopole shift is negative as Q = -x (the term of the monopole field is positive but it becomes negative from this term)