If you're looking for a properly symbolized legend in ArcGIS for Windows Mobile, you will unfortunately have to write your own implementation using the LegendItem class to render the swatches.
As a start, you would do something similar to this...
size = new Size(25, 25);
IList<LegendItem> legendList = layer.Renderer.GetLegendSwatches(YOUR_PANEL.BackColor, size.Width, size.Height);
if (legendList != null)
foreach (LegendItem legendSwatch in legendList)
PictureBox picture = new PictureBox();
picture.Size = size;
picture.Image = legendSwatch.Image;
picture.Location = // .......