In some areas the scroll wheel scrolls way too much. It's confusing and it makes the user miss things. Those areas are for example:
-the wallpapers in Desktop Settings
-Window Decorations and Desktop Theme in Workspace Appearance in System Settings
-The list in icon selection dialog

And other similar areas. I think it might be lists that contain icons/images.

Some of these areas ignore "Mouse wheel scrolls by n lines" setting and some don't. This setting is set to 3 lines, which is the default and normal in most cases.

I don't know if this is a local problem or should I report it on bugs.kde.org.

I can confirm that it does appear to scroll more than is expected. I believe this is because "3 lines" means to these views "3 rows" - which in the case of the wallpaper selection at least can be a good section of the screen scrolling by.