Hi,
I had to build a MASCARET model for EDF (imposed for a dam break study) and I used PreCourlis to build the cross sections on QGIS and export geometry file for Fudaa-Mascaret.
It worked pretty well even if a lot of work had to be done on Fudaa after this step (bief, model structure, weir, computation options, ...) but at least your topography part is done.
This is my personnal opinion but for 1D model, HECRAS is more advanced AND user friendly than Mascaret. Again, personnal opinion.