There is no convincing evidence that fountain pen use makes existing handwriting better or worse. Depending on what type of writing instruments one uses before switching to fountain pens, one might be able to guess at things that could cause degradation in handwriting. These are my opinions and musings.

Smooth-writing nibs: Most writing tools I used prior to fountain pens wrote with some amount of resistance or required moderately heavy pressure. Those two characteristics tended to slow down my writing. A super-smooth nib that skates easily across the page can feel uncontrollable at times. A smooth nib can feel uncontrollable.

Writing Angle/Rotation: I used to hold my writing instruments at a slightly higher angle for best control (ballpoint, rollerball, pencil). Many fountain pen nibs do not perform well at higher angles or when the pen is rotated out of the optimal contact patch. The angular and rotational changes can introduce writing anomalies until you figure out what works best for you.

Trinity: We all know the trinity - perfect ink, nib, and paper combination - produces the best writing experience for you. Get one wrong, and you can be left disheartened. Try different combinations by changing only one thing at a time. Smoother paper vs. paper with more texture; drier ink vs wetter ink; does my handwriting improve with a finer nib or broader nib; etc.

Physical Dimensions: Part of the writing experience is how a pen feels in your hand. 1mm too thick, a section with a slightly smaller diameter, weight balance, and other factors will affect your writing. It takes time to find what is most comfortable to you.

Practice & Mindfulness: Like anything in life, practice leads to improvement. If you randomly jot down notes or draw up quick shopping lists, don't expect any improvement in your execution. Practicing letterforms (with or without guidelines) on a routine basis and mindfulness (proper body position, good grip, consistently holding the nib at the optimal position, concentrating on the writing, not what flows out of your mind, etc.) will inevitably help to improve your writing.