Being an animal or creature/Zoan I’m thinking the personality of the creature is influencing Luffy just a bit. Like Chopper and what I’m certain is an awakened/semi awakened state, Monster Point, Luffy will eventually learn how to handle Gear 5 better in the future. Both physically and mentally of course.
Brook’s social skills deteriorated while being alone for so long. Upon meeting the crew and joining in, you could see him slowly becoming a little more tolerable in social settings along with the lessening of his Skull Jokes.
When Sanji returned from the other Isle of Women, his head was all messed up when around women, bleeding all over the place. Then over the course of Sabaody and Fishman Island he slowly started to go back to normal.
Those are just two of many examples that I think Luffy will follow. His mind isn’t used to the entity that is Zoan Nika and its personality seeps into Luffy a bit. It doesn’t seem to control Luffy’s decisions but it’s just a new feeling Luffy hasn’t gotten used to yet.
I want to say like Gear 2, it won’t tax or hurt Luffy’s body in the future but a part of me is wondering if this rapid withering could actually result into something awful for Luffy in the future. The mind part however, I’m betting Luffy will always laugh while using Gear 5 but as time goes on those hits will feel less potent and we’ll see less and less of the doubling over and cracking up uncontrollably.