import arcpy
mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd, "*")[0]     # grab 1st dataframe in map
#df = arcpy.mapping.ListDataFrames(mxd, "Main")[0] # grab dataframe named 'Main'
arcpy.MakeMosaicLayer_management(
    'Orthophotos',  # input mosaic
    'test_mosaic',  # output mosaic
    '#',            # Optional WHERE clause
    df.extent       # clipping box coordinates
    )