You could, but it's more work as it takes a lot of digging. You could also have flat raised beds setup the same way, with logs and branches and hay - it's a lot

You could, but it's more work as it takes a lot of digging. You could also have flat raised beds setup the same way, with logs and branches and hay - it's a lot more common thing actually. However small slopes and mounds on their own have a number of advantages: catching more heat when Sun is low on the horizon, and less in the summer/midday, and better drainage of any excess water - soil will never turn into mud, so less chances of roots rotting when there's too much rain - and there's also more air circulation around plants. However for smaller plants to get enough of the sun it takes more planning how to position them, as one side is always in shade. Slopes catch less rain, so in dry areas you need densely planted small plants or grass to catch it, and water that runs down needs to be channeled to accumulate on the bottom of the slope. Also, when planting and doing composting at the same time one needs to be careful that decomposition doesn't go too fast because the heat released in the process can hurt the roots, too.