I think so, if your word stronger mean physically.
Luffy - From what he says to Blueno and the second gear, he doesn't improved his strength for sure, even from the beginning of Water 7 arc (Aokiji) to the latest chapter.
Zoro - He is a person who always vow to become stronger. An obvious evidence of his is the attack that possesses the ability to cut steel. At first, he can't cut steel. His first technique that cut steel is Shishi Sonson from Mr.1's fight. Then skip to Karasumagari which he cut those cannonballs at Franky House, to Ie Raishomon which he used to cut one whole Puffing Tom car.
Nami - By her strength alone, Nami doesn't seem so strong ever since the beginning. It's her weapon that improve her strength in fights. From the normal mace stick, she changes to Clima Tacts in Alabasta. Clima Tact possesses the ability of making heat, cold and electricity. Combined with her intelligence, she creates all kind of natural things, like lightning bolt(Thunderbolt Tempo) or strong wind(Cyclone Tempo). Then she changes to use Perfect Clima Tacts just at Eneas Lobby. Perfect Clima Tact's attack is surely wider, faster and more powerful by looking at the only attack she perform so far with it, Thunderbolt Tempo, plus the dials inside the tacts make it stronger.
Usopp - The clearest improvement Usopp has is his courage and intelligence. He comes up with all sorts of new weapons, like Usopp Pound. He also increase his ability in fighting with dials. His courage is clearly showed to be improve since his challenge to Luffy for a fight.
Sanji - His kicking techniques improve constantly. At first, he can't break steel, but on Puffing Tom, he almost overcome Blueno's Tekkai. Most of his later technique use lots of momentum, like Concasse, Troseme Hashe and Bouchette. He seems to be improving as well (at least for the sake of being able to catch up with Zoro.)
Chopper - At the first met, Chopper seems to be stronger than Luffy with his arm point able to break iron since his first fight. Chopper's attack seems to be better, from Kokute Roseo to Kokute Cross. He is braver from the beginning, for that he wants to be a better pirate and want to help his friends with all his might.
Robin - Robin is hard to decide, since she comes the latest of the crew. Robin is already strong, but from her attack, she seems to improve as well, from Crush alone to Delphinium then Crush.