I think Lisa is awesome. Also she's one of the only "free" female characters in the game. Literally 2/3 of the females, were limited time/episode tie ins/holiday/and or cost donuts. She's smart. I don't have a problem with that.
Couldn't we say the exact same thing about Homer though? He's a one trick pony. He's dumb. The end. They try to redeem him on occasion but overall he's a terrible husband, a terrible father, lazy, an alcoholic, willfully ignorant. I mean everything he does revolves around him being "dumb". The schtick has been old for about 13 seasons. The main time he's funny is when he surprisingly knows something he shouldn't but even that wears thin.
So I can seen how Lisa can get old but at least she's intelligent with good ideals.
(I will say though that The Simpson's has a rich tapestry of characters and it should be no problem to bring different ones in to do these quests)