Leblanc is NOT dumb. You have to be seasoned enough to carry games because she has the LOWEST base health among the assassins, and she has to approach enemies closely to deal damage. She can poof back but she melts away if she is CC'ed during W. Also, because she is bad at farming, so if she loses lane badly it's hard to make a come back. Also she doens' have chance to build pure defensive items like banshee or GA because she is a main burst dealer in her team, and getting zero AP defense items causes lots of deal loss. It's even harder for LB if enemy team has bruiser and tanks(meaning her team failed to win early).
Bad LB players are one of the worst frustration; they don't know combo, good item choices, and timing to jump into the enemies. The team ends up having 4 vs 5 fights. For example, I had LB buying Athene and liandry by the 30 minutes mark....we lost that game because she was our main dealer and wwithout someone to burst the enemeis down, we lost very quicky