Yeah, as far as I know it's just a matter of Tapastic choosing your comic to be spotlighted and contacting you if they want to feature you. I don't think there's any specifications you have to meet, other than creating a comic that is so good (in whatever way it is good) that Tapastic thinks you would be a good advertisement for them.
Because I mean, the front page isn't just about getting exposure for the comics that are featured -- it also means that anyone who's new to Tapastic sees that comic first thing as an example of the kind of content Tapastic offers. Tapas staff is gonna want to pick works that are really representative of the kind of website they want this to be!