  We study the influence of the interface quality of
Pt/Y$_3$Fe$_5$O$_{12}$(111) hybrids on their spin Hall magnetoresistance. This
is achieved by exposing Y$_3$Fe$_5$O$_{12}$(111) single crystal substrates to
different well-defined surface treatments prior to the Pt deposition. The
quality of the Y$_3$Fe$_5$O$_{12}$(YIG) surface, the Pt/YIG interface and the
Pt layer is monitored \textit{in-situ} by reflection high-energy electron
diffraction and Auger electron spectroscopy as well as \textit{ex-situ} by
atomic force microscopy and x-ray diffraction. To identify the impact of the
different surface treatments on the spin Hall magnetoresistance,
angle-dependent magnetoresistance measurements are carried out at room
temperature. The largest spin Hall magnetoresistance is found in Pt/YIG
fabricated by a two-step surface treatment consisting of a "piranha" etch
process followed by an annealing step at $500^\circ$C in pure oxygen
atmosphere. Our data suggest that the small SMR in Pt/YIG without any surface
treatments of the YIG substrate prior to Pt deposition is caused by a
considerable carbon agglomeration at the Y$_3$Fe$_5$O$_{12}$ surface.
