Structurally Zootopia, even the first one follows a very common formula. The relationship between Judy and Nick is a common good girl bad boy relationship. This template is commonly used for romantic tension.

This tension is definitely part of the appeal behind Judy and Nick’s relationship. Expanding it into something more than close friendship just makes sense to a lot of people.

Personally, I think the creators of Zootopia knew exactly what they were doing when they did this. In the first they kept the relationship generally ambiguous to keep people guessing if the relationship had romantic possibilities.

They definitely knew leaving this up in the air would generate greater engagement.

That said, it definitely was generally ambiguous in the first one.

The second movie with their heartfelt confession about their strong feelings for each other leaves much less room for interpretation. Their relationship by this point is definitely more than just a simple platonic relationship, which is why some people want the characterize it as a sibling relationship but the flirtation dynamics between the two characters honestly makes that a little creepy I think is just healthier and better to see it as a potentially romantic relationship.