In my experience, yes. Blitz is great and fun, but you do not improve from playing just blitz. If you play classical and improve, your blitz rating will follow
Is it good to play only blitz?



Playing chess with more time, like in rapid or classical games, lets you think more about your moves. This helps you understand better how to plan ahead, see tactical patterns, and think about the long-term in the game. But it's also okay to play fast games for fun. Mixing both can be good. Playing some not-so-fast games, like rapid chess, might be a nice balance between getting better and having fun.
In the last few months I've ONLY played blitz and BULLET, but I've discovered that many say it's useless and that only RAPID makes you improve. Is it true?