Practice makes perfect.... or not?


Yes. As Erik mentioned playing lots of blitz/bullet has negative long term effects on your playing ability.
Slower time controls, endgame study, tactics training, and looking at positions and variations (problem solving and visualizing) in a physical board have all proven to steadily improve chess.

(I should note that it was actually blitz chess, but I did a brain fart and said bullet)

The reason is that in blitz games you don't have enough time to think and make a correct judgement on the position. You basically play off "instinct" and memory of similar positions. It is possible that one makes improvement but it will be a very slow improvement.
Blitz improves tactics or short term combination. Normal speed chess improves Strategy or long term plans and positional advantages (which in turn also improve tactics).

(I should note that it was actually blitz chess, but I did a brain fart and said bullet)
It's very common that people waste years playing speed chess and never break 1000 rating. If you really want to improve, I'd quit playing it altogether.

In reality, practice makes permanent. If you play bullet without really thinking, not looking at why you lost, not looking up better moves, not digging into all the tactics you missed ... then you'll get really good at playing bullet without thinking.
It takes more than just mindless practice, it takes deliberate practice, of intentionally trying something, seeing the results and then using that to improve. Suffice to say, the vast majority of bullet players do not do this and play just for the rush. There's nothing wrong with that, but you can't then turn around and complain you aren't improving.
Playing too much can be bad, because you try to "hold your level" (or even improve), but this can be deceiving. Especially playing too much Blitz (or even worse: Bullet) can be a bad habit.
I switched to 15 10 recently (instead of playing Blitz), but i am quite bad at it (although my OTB Rating is not that bad in really long games).
I think: by playing alone (without analysing afterwards, and without training apart from playing) progress is not easy (or even impossible).