Emphatically we can say that cleaning cat ears routinely is not required or even a good idea. The general advice is leave them alone.
However, routinely checking cat ears for obvious signs of health problems is sensible. Classic ear problems are ear mites and abscesses (from fighting). Ear mites can cause a bad odor to emanate from the ear. Abscesses are obvious – swelling and skin lesion.
Ears should have some wax. This is normal. Excessive wax is not. When wax is excessive or when there is dirt inside the ear, cleaning them may be necessary. Personally, I have never had to clean my cats’ ears for these reasons. My guess it that they are fairly rare conditions. I do check them regularly though.
Two immediate issues arise with ear cleaning, (1) cats normally won’t tolerate it that well. Gentle restraint may be necessary and patience. A piecemeal, bit by bit cleaning is probably the way. Forcing the matter is not a good idea (2) using cotton tipped swabs is fine to clean between the folds of the ear flap but the swap should not be pushed down the ear canal.
Using a water dampened cotton ball or cotton tipped swab will be adequate most times. Special ear cleaning solutions are available from a veterinarian. Alcohol, ether and other solvents should not be used as they may inflame the ear and cause pain. A Cleaning solution available in USA is Oti-Clens:
And in the UK Epi-Otic Ear Cleaner for Cats and Dogs (Size: 60ml Bottle) is available on Amazon too: