Well Amy, it all depends on your soil and climate but in most cases wholly thyme and creeping thyme do not make a solid weed blocking groundcover on a slope. Thymes need lots of sun, dry winters and do best when growing in cracks and rocks. I do have a thyme groundcover patio and walkway in my Western Washington garden but it is successful because it grows in two inches of sharp gravel with the fines and fills in around large flat stones. Still it needs some hand weeding every spring. (You can find my thyme patio on YouTube using my name) You did not mention where you live but for a sunny slope I would recommend evergreen shrubs like cotoneaster Lo-fast, creeping blue rug juniper, or spiraea and barberry shrubs. It is most practical to use shrubs and boulders on a large piece of sloped land. Another idea is to ask your local Master Gardeners or at a local nursery  to make sure the plants I recommend fit your particular climate zone. Keep growing, Marianne Binetti