You should post some pictures of your forms and a general plan of how you plan to build the mini ramp if you want an accurate answer. Dirt is fine as fill if it is not supporting your ramp or if the ramp is going to be poured as one solid piece that will hold itself together with steel. Basically the bottom line is that raw, uncompacted dirt has little 'structural' value and will probably erode over time because of wind or rain, but depending on how you are building the ramp it might not matter.