The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally doped SmFeAsO_0.88F_0.12 iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents (x) by means of muon-spin spectroscopy (μSR), both in zero and in transverse fields, to probe the magnetic and the superconducting order, respectively. The pure superconducting phase (at x < 0.03) is replaced by a crossover region at intermediate Mn values (0.03 ≤ x < 0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x = 0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of T_c and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors (M. N. Gastiasoro et al., Phys. Rev. Lett. 117, 257002 (2016)), which includes correlation-enhanced RKKY couplings between the impurity moments.