Since for some odd reason the news about Twisted Metal supporting custom skins has not been reported yet on D-Toid (and most other outlets) I decided to tell you myself. Jaffe reveals this during a interview with IGN which you can find here
(it starts at 2:16:00)
You can create these custom skins on the TM website or if you have artistic/technical tallent you can use the released templates to create your own skins and them upload them to the TM website. These skins can them be used online. They can be shared to everyone or be locked to just yourself or your clan. Here are some skins ESP created.
Source: Jaffe's Twitter https://twitter.com/#!/davidscottjaffe