For me, someone who is lame is no fun and "uncool". To say that someone is a sheep means that he does what everyone else does out of cowardice. I hope that helps!

Kristen is right. I guess "he is so lame" can also be translated as "he is slow" (/lazy /sleepy). Literally it means: He can't walk. I think someone is disappointed because "he" didn't respond fast enough. Or not as expected. The "sheep" might be a consequence (/result) of being "lame". D. who sometimes is lame...