Is copper sulphate a mixture or an element?

It is neither a mixture nor an element.

Copper sulphate (CuSO4) is a COMPOUND.

A compound is a substance consisting of atoms, which are combined together using the electrons in their outer most energy shells.

As a compound it is composed of atoms of three different elements. viz.,

1 atom x copper (Cu)

1 atom x Sulphur (S)

4 atoms x Oxygen (O)

Mixtures do NOT use their respective electrons.