What makes a catchy song bad?
Catchy songs aren't bad.
Bad songs are bad. Some of those bad songs just happen to be catchy, and it just so happens that many of these bad songs that are catchy just happen to be popular songs these days.
There have been some good songs that have been catchy as well (and popular).
(Note, if something is stated to be bad or good in the post, it is entirely my opinion, not meant to be taken as an objective statement)