While mayonnaise is not considered to be particularly dangerous for dogs, it is fairly unhealthy for both people and animals due to its high fat content. If you want to give your dog mayonnaise it is highly recommended that you avoid giving it too much over a long period of time as it could cause problems with its healthy.
Even people should avoid adding too much mayonnaise to their diet. Those who want to give mayonnaise to a pet will want to select the light or fat-free variety so it is not quite as unhealthy for them. While it is true that this condiment may have some vitamins that are good for you, it does have a fairly high amount of saturated fat.