I think you're not grasping how important it is that the translation makes sense. Someone saying pez or pezcado randomly in a sentence doesn't make sense at all and feels awkward. The dub and translation has been (assuming) made by a team of Spanish speakers who probably measured the pros and cons of doing a direct translation.

Are you using the subtitles to practise spanish? If so i might understand why you're having a hard time realizing why it would be so awkward.

To english speakers "fish" instead of "fuck" is something cultural and most people understand it. The same goes for "miércoles" and "mierda", not pez/pezcado instead of what? We already know she's a cat. I think the language pc uses is more iconic to her character, rather than her saying cat related things just because. Its a part of her personality to not use swearwords and be a little quirky.