I agree that towel animals are a random event, and depends on your mousekeeper. From our experience, I don't think the tip can influence if you get a towel animal or not.
Over our past 5 trips, staying at the All Star resorts, we have always tipped 4 dollars a day in a mousekeeping envelope. Just DS & I on our trips.
1st Year - nothing
2nd year - DS's toys were occasionally arranged watching TV, reading a book, peeping through the window....
3rd year - 2/3 towel animals
4th year - nothing
Last year !!!!! Towel animals almost every day. We had elephants, monkeys hanging from the clothes rail, turtles, rabbits, swans, Mickey head, basket of flowers - it was amazing !!!
During our second week, the towel animals stopped for 2 days, so we assumed the mousekeeper had changed. On the 3rd day, a British bulldog soft toy appeared amongst DS's other toys

and the towel animals appeared daily to the end of our trip.
We never met the mousekeeper - but she certainly added a huge amount of magic to our holiday.
Sue