The defender scored in the 2-1 FA Cup win at Hull City on Saturday.
"Working with Fikayo up at Derby, we saw the things we needed to improve, even things like personality-wise, as he was quite quiet," said Lampard after the centre-back scored his second goal of the season.
"Now you see a different change in his personality and his development. He's played for his country and he's playing regularly for us and doing very well. I'm delighted with his progress.
"I'm delighted that we scored from a set-piece – we don't score enough from them and we work on them a lot. Sometimes it's the execution, sometimes it's the delivery; normally those doing the heading blame the delivery and vice versa.
"We got it right and that's pleasing. They can tip games. It's something we work on and we need to do better."