The checkstyle plugin decorators merge with those java decorators. This is bad because when doing a refactoring legacy code (with thousand of problems) does not know what is validation of checkstyle or java file broken when looking in the package explorer.
It would be usual there was an option to not use the decorators, so we could only navigate the problems on Makers and when a refactoring breaking the code would be easier to see it, since it would not be lost in the middle of thousands decorators in Package Explorer.
Unfortunately the checkstyle plugin doesn't add anything to the package explorer icons by itself.
The decorators are provided by Eclipse itself, based on whether a file contains error/warning/info level markers - of any type.
This behaviour can - to my knowledge - not be customized, hence this RFE is impossible to implement.
As explained the request is impossible to implement for this project.