I have had trouble with brassica germination in the past when I tried to work through too much thatch. Some is good, but you have to get your seed on /in the soil and not have such a mat of grass that it suppresses germination. It is hard to define "too much", but I do recognize the look of it now.
If you can get any type of a drag in there to scrape it up, I would do it. One time I drug it until it was clumped up and either pushed it off the field with the tractor bucket OR just left the clumps and planted around them. You could also think about burning the thatch off.