From your picture, it looks like wax build-up to me, but better to have the vet check to make sure, as mites can cause a lot of aggravation and spread to your other cats and can lead to an excess of wax.
It's difficult for cats to clean their own ears very well, especially kittens, until they learn to wet their forepaw and rub it in their ear. Mom cats clean their ears for them. Some cats don't seem to learn to do it themselves, or rely on their cat friends to wash out their ears for them. My neuter never cleans his half-sister's ears, so I have to do it for her as she doesn't do it for herself, whereas she always cleans his ears.