I don't know about avocado as a specific ingredient - but personally I would be wary of any product that doesn't specifically list cats on the label - cats are much more sensitive to chemicals than dogs & horses, and have more sensitive skin, too. If a product is safe for cats, it will usually mention on the label that it is safe for cats, too. If it doesn't, I wouldn't risk it - there's a chance it could make your kitty sick.
There are lots of pet shampoos available that are safe for cats at the pet store - just check the labels to make sure they list the product as safe for cats, and stay away from Tea Tree oil & essential oils, like Helen suggested.
If you are unsure about the ingredients, you can always check with a vet to be safe.