It's by no means dead or it wouldn't charge at all, but it is starting to show signs of not being able to hold a full charge, and that's very common as it happens to all rechargeable batteries eventually. No need to panic if it's managing 90% but it will gradually get worse.
It's your call whether you decide to replace it now or just keep an eye on it, but it's still managing 90% so I wouldn't scrap it yet.