Hermione was still human - just with catlike features and fur.
Hermione never actually turned into a cat, changing species - she simply mutated, becoming more catlike in appearance.
“Hermione let her robes fall and Ron backed into the sink.
Her face was covered in black fur. Her eyes had gone yellow and there were long pointed ears poking through her hair.
‘It was a c-cat hair!’ she howled. ‘M-Millicent Bulstrode m-must have a cat! And the P-Potion isn’t supposed to be used for animal transformations!”
- Harry Potter and the Chamber of Secrets, Chapter 12 (The Polyjuice Potion)
In addition, when used with cat hair, it didn’t wear off like it was meant to.
Polyjuice Potion is supposed to wear off on its own after an hour. However, Hermione’s Polyjuice cat transformation took weeks to cure, even under the care of Madam Pomfrey.
“Hermione remained in the hospital wing for several weeks. There was a flurry of rumour about her disappearance when the rest of the school arrived back from their Christmas holidays, because of course everyone thought that she had been attacked. So many students filed past the hospital wing trying to catch a glimpse of her that Madam Pomfrey took out her curtains again and placed them around Hermione’s bed, to spare her the shame of being seen with a furry face.”
- Harry Potter and the Chamber of Secrets, Chapter 13 (The Very Secret Diary)
It’s very clearly not a successful transformation into a cat, and in fact, this sort of result seems to be exactly why Polyjuice Potion is said not to work to change species - because it doesn’t fully transform a human into an animal, and it requires medical care to change fully back.