Dust on the viewfinder eyepiece usually does not affect image quality, because it is not on the sensor or in the image path recorded by the camera. However, it can be distracting when composing, so it’s reasonable to clean it.

Use a tool intended for small optical surfaces, such as a viewfinder/lens cleaning pen or another dry cleaning method. Be careful not to get the camera wet, especially if it is not weather-sealed. In general, this is more of a viewing annoyance than a photo-quality problem.

So: if it doesn’t bother you, you can ignore it. If it is distracting, clean the eyepiece carefully with proper optical cleaning tools.