There are artifacts in the repository that don't have POMs. They come from the Maven 1 repositories of our partners (Apache Software Foundation, Codehaus,...). We know it but can't do anything unless you provide a POM for it or you ask the project in question to add the POM when they add the artifacts.
We don't change dependencies in POMs already in the repository anymore as builds need to be reproducible.
You need to contact the original publisher of the metadata to make sure in next versions it will be fixed or improved before getting it into the repository.
For any other types of issues related to metadata in the repository, open an issue at JIRA MEV with the relevant information and explain the reasons why it is an issue.