Can you be a good corporate lawyer with average writing skills?
No. You will never survive as a first year associate if you can’t write well. That said, legal writing is not really the same as any other writing. Also, there are many lawyers out there who aren’t such great writers, either. If you’re an average legal writer compared to all the other legal writers out there, you should be okay. If you’re just an average writer, period, you may be a better legal writer than you think once you’re properly trained. There are certainly upper-level positions in firms that require less writing than others, but you’ll have to be a pretty good writer to make it that far.