This is a pun on the word "point". In mathematics, a decimal number has a point, or dot, that indicates that it is not a whole number. The phrase "have a point" means that you have a good reason to support an argument. It can also mean that something has a purpose. In the joke, the person might have disliked maths because they thought it had no purpose, no good reason to be used. But because they saw the point, or value, in decimals, they like the subject now. They also saw the literal point, or dot, in the decimal number.
I used to hate maths but then I realised decimals have a point .