There is a big difference (for example, butter is widely used in the north, while oil is widely used in the south-central part). But in cities or villages, there are different versions of the same dish.
Italy's history and geography make the language (dialect), tradition, food and other aspects of Italy's "nature" very detailed. You move to a place a few kilometers away from your village and find that you haven't heard some words, some food you haven't eaten, and some traditions you haven't practiced. This may be the case all over the world, but I don't have the knowledge to turn this assumption into a statement.
In addition, even in the same small village, your family menu will be slightly different. For example, my mother and my mother-in-law live 7 kilometers away, but there are many different ways to cook the same dish.
Pizza itself is different all over Italy.
For a foreigner, it is "Italian food"; For us, it is a galaxy composed of different things, experiences, memories and perceptions. Even those who are unwilling to fight for these trivial things still admit that these differences are a deep part of our identity, whether as individuals or as members of a specific group.
When we discuss recipes, it is not because we are looking for a stupid reason to quarrel, but because when we grow up, we are strongly aware of the seemingly insignificant differences between foods from different regions, different towns, different villages and different families.
So, in the end, the Italian's reaction to deep-dish pizza depends on whether his/her personal experience of food is appropriate.
After all, we're not talking about pineapple pizza: it's a disgusting thing and worth lighting the hottest fire in the deepest part of hell. They don't think about anything, because they ignore its existence, which is lucky for those who prepare it (otherwise they have been caught and executed by the international pizza police).
Seriously, this has nothing to do with pizza, but people's fascination with ridiculously thick toppings. The closest food that Italians think is acceptable is pie (for the Chronicle, ketchup is put in the top cup, not ketchup: usually it is already spread on it-but in this case, it is served separately).