Yes, all mammals have belly buttons whether we are able to see them or not. Cats, in this case, have belly buttons, too, considering they are also a mammal.
Keep in mind that just because we are unable to see them ourselves, it does not mean they are not there.
More often than not, looking for a belly button on any mammal in general, not just on a cat, will prove to be unsuccessful. The belly button of a cat does not develop like a humans does, meaning they do not have an “innie” or “outie”.