You will find that it is perfectly acceptable to give your cats a little bit of mayonnaise once in a while, especially when it is mixed with salmon. It will however be important not to give them too much of it because of the fact that it is relatively high in fat. If you want to avoid fattening up your cat with mayonnaise you will most likely wan to feed them the light variety which does not contain as many calories.
There are even completely fat-free versions of mayonnaise which you can get for you or your cat, making it a great snack for them. Because mayonnaise is not exactly considered to be healthy though, you will want to make sure to give it to them in moderation.