What is a Catholic hymn?
Over the past several years, the U.S. Bishops have been turning their attention to ensuring a beautiful and reverent celebration of the Mass in accord with the Church's tradition. For instance, in 2011 they finished a project to retranslate the Mass so that it was more faithful to its Latin original text, and included the traditional language of our faith.
They have recently turned their attention to sacred music in the liturgy. The Church's tradition has practically from the beginning preferred chant as the music that belongs to the Mass, mostly coming from Scripture. In the 1960s and 1970s, as an alternative, but never the ideal, they allowed popular hymns.
However, the problem has been that many of these hymns are not Catholic in nature. That is, they contradict our doctrine, or they use vocabulary that is alien to our faith.
Their committee on doctrine (teaching) has recently recommended a review of hymns used in the Mass, and specifically named some that were problematic. So even if they are "catchy tunes" they need to be eliminated from the celebration of the Eucharist.
You might notice some slight adjustments in our parishes as a result.
Here's an article on this topic that from one of the Catholic news agencies.