in personal experience, i believe there are a few good times to buy DLC, but for Mario Kart 8 there are just two i think, and that would depend on your patience and your will to spend your money xD. I guess the option is up to you in the end, whether you wish to spend or save, let me explain:
The first option, that many hardcore Nintendo fans would go with, is to buy the DLC on release or soon after, this is so that they get to experience everything that Nintendo has to offer, you get cool new tracks and karts and parts, it's a blast for those who can afford to keep up with Nintendo's DLC creation XD.
The second option, that people who are on a budget, choose to wait for a sale in which they can get the DLC at a discounted price, this is not a super common option, but many people do it, as they can "afford" to wait and just enjoy what they have before getting the DLC at a discount.
In the end, if you really want to get the new DLC and can afford to get it, you should get it, if you are trying to save money then you should wait xD