It's about little cats that live in bananas. "Nya(n)" is the sound that cats make in Japanese.
I watched it because it sounded odd and the episodes are only three minutes long. Was it worth it? Not really. It's aimed at a pretty young audience and it's just cartoon characters being cute in a plastic world of cel-traced CGI animation. They can't even talk. They certainly don't have human-level intelligence. They're a lot like real cats, oddly, apart from a few human personality traits and some tweaks to make them cute (rather than realistic).
Bananya himself likes eating cookies, for instance, and can be bribed with sweet things into becoming a mouse's friend, although that's no guarantee that he won't then lapse back into normal mouse-chasing behaviour. Other banana-cats include Bananyako (the token girl, like Smurfette), Daddy Bananya (balding, bespectacled, reads newspapers), Long-Haired Bananya (narcissist) and assorted Bananyas who are basically defined by their appearance. Tiger Bananya has tiger stripes, Mackerel Bananya has a mackerel-like colour scheme, etc. They actually have a perfectly workable level of characterisation, all things considered, but that's still not a huge amount given that there are a dozen or so of them and these are only three-minute episodes.
Bananyako vaguely annoyed me. She's the token girl in a very large group of cartoon cats, so of course she's being written as exaggeratedly girly and has hobbies like "research make-up every day". (In fairness we meet another female Bananya at the supermarket in ep.6, when Bananya sneaks into someone's shopping basket and hence leaves the house. She's foreign. However she's also even more of a caricature of femininity than Bananyako, being basically an 18th century pre-revolution French princess.) In the show's defence, though, it would be possible to argue that we don't know the genders of most of the Bananyas, since Japanese doesn't have gendered pronouns. It's only the English subtitles that call everyone "him". It's conceivable that the cast is secretly gender-balanced, with Bananyako and Marie Antoinette standing out simply because they like to primp themselves up.
(Bananyako even has an unflattering character design. She's supposed to have luxurious fur like a Persian, but in this simplified art style that translates as "needs a comb". There's also a bit in ep.10 where the animation can't communicate what's happening in the scene and so it has to be done verbally, by the narrator. Supposedly the cats' claws are in danger of popping a balloon.)
I watched the show, though. It scrapes in at the lowest level of "a bit amusing", if you've got low standards and nothing better to do. We're not the target audience, though. The Bananyas do aimless cat things. They sit looking at the TV while the narrator wonders if they understand it. They sharpen their claws on chair legs. They climb into the fridge and squeeze themselves into narrow places. They also like mischief, allegedly. We're told this on a few occasions. Most of this "mischief" is nothing of the sort and it's almost pathetic to see it described as such, but in ep.13 they trash everything that's been laid out for a child's birthday. (In fairness they also decorate the cake, but very badly.)
It's cute. That's all it is, really. Animals do cute things while saying "nya" to each other, although I'm not sure I'd even call it that cute, personally. They live in kitchens and pull up their peels when they hear a human coming, so surely they're going to get eaten sometimes? Bananya's ambition is to become a chocolate-coated banana. Again, eaten. However this is a universe where a banana-sized cat in a banana skin can run around the floor in the supermarket, then climb up on the cash register... and nobody notices. Realistic considerations do not apply.
It's trying to be cute. That's a better way of putting it. It's also gentle, aimed at children and almost content-free, although it does sometimes make modest efforts to base its short episodes around its characters' personalities.