Which export method did you use when exporting from FCP?
Normally you'd "send" to Compressor and choose the appropriate DVD preset - or - export a self-contained movie (File->Export->QuickTime Movie), then import the resulting file into Compressor.
I'm guessing that you're looking at an interlaced video on a progressive screen...
If this is the case, you'll want to de-interlace the footage in Compressor while transcoding to MPEG-2.
also, since you're scaling the material, you may get better quality by customizing the Compressor preset. Enable Frame controls, and change the resizing filter to best. You can do a test of a short section by putting and in and out in the preview window.