At the trial stage, you can argue matters of fact. Stuff like "my client was at home in bed when the events occurred" or "those teeth marks don't evem match his dog!"
Since we are now at the appeal stage, barring something huge, all the facts the trial court decided were true have to be assumed true. So he can't now argue that Weev didn't actually access AT&Ts servers. All he can discuss now is whether the court correctly applied the law to the facts it determined at trial, and that means...
...that it won't hamper Kerr at all. The facts here aren't disputed; everyone agrees on what Weev did, how AT&Ts servers were configured, where Weev was, where the servers were, how AT&T responded to the breach. The dispute is entirely down to how the court applied the law (or indeed, whether it was even in the right court), and that's stuff which is best addressed (in some ways, only addressable) at the appeal level.
TL;DR: It doesn't make Kerr's job harder at all; in fact he can only do his job at the appeal stage, as his concerns are very much with the trial courts decision, not with the facts the court based that decision on.
Normally appeals courts only handle the question of whether the lower court applied the law correctly, and assume that the lower court interpreted the facts right. This is because normally (there are a few VERY rare exceptions) it is not legally permitted to appeal on the grounds that the court or the jury got the facts wrong... only on the grounds that the law was applied incorrectly.
All this said, Kerr is only consulting/helping out AFAIK. He is not the arguing attorney (I hope). He's not really a litigator (he has about 3 years of experience in it, most of it from very early days of his career).
I am doubtful that his expertise would have really mattered at the trial stage. You can either convince a judge of something, or you can't.