  Chemical substitution is commonly used to explore new ground states in
materials, yet the role of disorder is often overlooked. In Mn-substituted
BaFe$_{2}$As$_{2}$ (MnBFA), superconductivity (SC) is absent, despite being
observed for nominal hole-doped phases. Instead, a glassy magnetic phase
emerges, associated with the $S=5/2$ Mn local spins. In this work, we present a
comprehensive investigation of the electronic structure of MnBFA using
angle-resolved photoemission spectroscopy (ARPES). We find that Mn causes a
small and orbital-specific reduction of the electron pockets, only partially
disrupting nesting conditions. Based upon the analysis of the spectral
properties, we observe, for all bands, an increase in the electronic scattering
rate as a function of Mn content. This is interpreted as increasing band
incoherence, which we propose as the primary contributor to the suppression of
the magnetic order in MnBFA. This finding connects the MnBFA electronic band
structure properties to the glassy magnetic behavior observed in these
materials and suggests that SC is absent because of the collective magnetic
impurity behavior that scatters the Fe-derived excitations. Additionally, our
analysis shows that the binding energy ($E_{B}$) dependence of the imaginary
part of the self-energy [$\text{Im}\Sigma(E_{B})$] is best described by a
fractional scaling ($\text{Im}\Sigma(E_{B})\propto\sqrt{-E_{B}}$). These
results indicate that Mn tunes MnBFA into an electronic disordered phase
between the correlated Hund's metal in BaFe$_{2}$As$_{2}$ and the Hund's
insulator in BaMn$_{2}$As$_{2}$.
