When you draw a stroke with a drawing tablet, your stroke will not be perfectly smooth. There are many causes for it any many things you can do to address it.
The tablet and driver are components that can contribute to strokes that aren't as smooth as you would like
All tablets have a degree of diagonal wobble that can appear in your stroke.
Tablet sometimes exhibit a little bit of "noise" in their stroke. This noise is similar to wobble in that it deviates from a smooth line, but it does so in a more random way.
The surface of a tablet may be too smooth and this can cause the pen to easily "slip" away from your intended path.
You are user and your drawing style can affect the smoothness of a stroke
Your hand and arm and way of moving them over the tablet can cause imperfections in your stroke.
Drawing slower is more prone to introducing errors into your strokes.
Consider how you hold you pen. Different techniques of holding your pen can affect how smooth your strokes are.
Brad Colbow - How Pros Hold a Pencil 2024/05/28
Using a digital; pen on a very smooth tablet surface can result in the pen feeling "slippery" as you draw. And so the pen often seems to "slide away" from the intended path of the stroke you are trying to make. This is a common complain for iPads because an iPad's surface if very smooth glass.
Consider buying a protective sheet to increase the surface texture a bit. See: protective sheets.
Another way of increasing the surface texture and avoiding a slippery feeling is to use a felt nib. Many tablets now come with felt nibs, though they may not be installed in your pen by default.
Many people draw just by moving their wrist. Try instead drawing by moving your shoulders more.
Usually someone's hand can draw a smooth stroke easier at some angles than at others. For example, if you are right handed drawing a strong from bottom left to top right is easy but drawing from top left to bottom right is harder and the stroke less smooth. Rotate the application canvas so that you accommodate what your hand is better at doing.
Some people find that drawing a stroke toward themselves is easier to keep smooth than drawing away from themselves.
Generally speaking a faster stroke will result in a smoother stroke.
Occasionally some people enable mouse mode in their tablet driver. Mouse mode uses relative positioning and usually makes it harder to make a consistent stroke. Instead try disabling mouse mode.
If you are using a pen tablet, mismatched aspect rations between your pen tablet and your monitor will distort your strokes and make it harder to draw smoothly. Make sure you check for this and correct it. More here: Matching aspect ratios.
Please do check for this. Many people have been using their tablets for years with mismatched aspect ratios and when they make the ratios match it is a BIG DIFFERENCE in their ability to draw strokes correctly.
Use zoom to your advantage. The stroke is affected several things which are physical in nature. For example, a diagonal wobble may introduce a 1mm wobble as your draw. If you are drawing with your canvas zoomed out then 1mm accounts for a lot of pixels. If you zoom in more then the 1mm accounts for fewer pixels of wobble. So you can really minimize some effects my zooming in as much as possible for your stroke. This minimizes the effect of those disturbances and also forces you to draw with a longer stroke which itself will minimize errors.
Use brush smoothing in your applications.
Krita, under Tool Options, has 4 different brush smoothing options: None, Basic, Weighted, and Stabilizer.
Clip Studio Paint has several options
A setting called Stabilization that goes from 0 to 100. Where 100 adds the most amount of smoothing possible.
You can also separately use the Adjust by speed setting. It has two options: Increase stabilization when drawing slowly and increase stabilization when drawing quickly.
Some apps like Clip Studio Paint have the ability to "fix" strokes after they have been drawn.
The advantage to this technique is that it doesn't slow down your stroke as you draw.
The disadvantage to this technique is that you don't exactly know the path of your stroke until a moment after you draw the stroke. Also if you draw a sharp corner, post-correction techniques can somethings not recognize the corner and instead show it as a smooth corner.
Applications like Krita also support vector tools. If you are having problems with smooth strokes with you pen, the vector tools can produce perfectly smooth strokes.
Some applications, like ProCreate on the iPad, have a a shape detection feature. Roughly speaking after you draw a stroke and hold you pen still for a moment, it will recognize a line, or circle, or curve and create a perfect smooth version of your stroke.