Arguably, no one in the MCU is smarter than Tony Stark. He proved his intelligence time and time again, especially when he developed time travel and saved half of all life in the universe. He was a genius, but others still outsmarted him many times.
In general, Iron Man's biggest mistakes happened when he carelessly underestimated those around him. Whether it was a young boy he was mentoring or a global terrorist, Stark tended to let his ego get in the way of his interactions with other people. And if the other people were smart enough, they exploited those moments to get the upper hand on Stark's iron hands.