$\mathrm{Cu}^+$ ion is unstable in aqueous solution because it:

$\mathrm{2\,Cu^+(aq) \to Cu^{2+}(aq) + Cu(s)}$. The disproportionation is spontaneous because the hydration enthalpy released on forming $\mathrm{Cu^{2+}(aq)}$ more than compensates for the ionisation energy needed to take $\mathrm{Cu^+}$ to $\mathrm{Cu^{2+}}$. As a result $\mathrm{Cu^+}$ is only stable in low-dielectric environments such as in $\mathrm{Cu_2O}$ or with bulky ligands.