Mulch is not the proper solution. It is steep. Unstable--subject to erosion and landslide during heavy rain. It needs landscaping. Plant something. Anything! Some possibilities: a seeded ground cover like ajuga, a low-growing grass with dense roots, like blue fescue, (do not mow it), lirope--as Coleman suggested above, wildflowers, maybe mixed perennials, daylilly, iris, bush clover, lamium, myrtle, pachysandra, honeysuckle, euonymous, blue-rug juniper, various semi-invasive legume vines--crown vetch?
Accent this with a few boulders, flowering crabapple, plus clumps of ornamental grasses--canary grass, maybe. Complement this with a decorative art object--statue, fountain or garden gnome.
Purple fountain grass is good.
https://www.bhg.com/gardening/flowers/perennials/ornamental-grasses/
Be sure to include irrigation so that it will be easy to start the new plantings and so that thin-out during dry spells can be avoided. Fertilize every year to keep it dense.