Yes, it is a noun. In your sentence, it means the same thing as "favourite cartoons", that is, the cartoons people like very much. I could say, "I like ice cream! Chocolate is my favourite", using "favourite" as a noun, or I could say "Chocolate ice cream is my favourite ice cream"; here "favourite" is an adjective. In North American English, "favourite" is spelled "favorite", but it is the same word.