Unlevel lawn: You want a firm, level surface to lay your new sod over. It may start out level, but as the old grass breaks down it will turn out very uneven.
Weeds: Your old grass is not dead and it will work to grow up through your sod and produce a weedy looking lawn.
Harder to root: You want your sod to root very quickly. You want to place it against firm level ground and give it the shortest possible distance for the roots to grow. By laying the sod over an existing lawn, the roots have a much longer distance they need to grow.
If you want a top quality job, you can’t take the easy short-cut. Take up the old sod and rototil your soil and prepare it correctly. More on installing sod.