Tom tells Myrtle that Daisy is "a Catholic" as a way of demeaning her and making her seem inferior. In the social hierarchy of the time, Catholics were often looked down upon by the wealthy Protestant elite, and Tom uses this to belittle Daisy and distance himself from her. Additionally, Tom may be trying to emphasize his own superiority and power over Myrtle by making her feel like she is not worthy of association with someone from his social circle.
