I think Bingley´s intelligence is always underestimated by:
1- Darcy´s more serious ways, superior intelligence and subtly elder brother position
2- His own insecurity
3- His amiability
4- At least one adaptation to screen makes him look like an utter fool
So it´s easy to think him like a very mild spineless person, but he´s quite intelligent -if not, Darcy wouldn´t tolerate him, just that is quite a clue.
He gives intelligent remarks, but his kindness makes him look mild when others give snarky remarks that could pass as intelligence.
I´m quite a fan of Bingley ;)