Off the top of my head I can't think of anyone I knew from law school who applied for federal appellate clerkships and struck out. I certainly know many, many who weren't on LR and probably didn't have top grades who landed awesome federal appellate clerkships though of course grades and LR don't hurt. The big deal is which judges you can get offers from - there's a huge range in competitiveness from one individual judge to the next (and between circuits as well).