Uhm... I'm not 100% sure what you mean by
neko, since it is a really unspecific term... But I can't see how a fluffy tail would be a problem for that. I mean, sure, felines generally have thin tails, but not all of them.
There are some cat breeds that have fluffy tails, like
this or
this. From the bigger cats,
snow leopards have kind of fluffy tails.
And, as you're writing
fiction I think it's okay to use a creative licence and have her just be a neko with a fluffy tail. *shrug* After all, didn't Takahashi do the same? At least I'd call Kirara's tails kinda fluffy, and she's a kitty.