Zombie Driver HD is on PSN? Had no idea! How is MUD vita?
Okay, finally can get back to you with some impressions on MUD.
First off, you control 4 racers. Each has a talent (not sure how important yet) and 4 attributes that you can earn money to power up from 1 to 10. Stats include stuff like how fast you can turn, boosting, and how easy you're thrown from your bike. So there's some level of 'progression' built in. I've only put in a couple stat points here or there but I definitely can feel them.
You can also unlock 'trick' cards (ability to do tricks) and gear (helmets) that I think will affect stats.
There seem to be 4 race types -- standard race, elimination race, trick score race, and head to head.
There are some in game ways to get boost -- you hold down brake and the pedal at the gate, and if you let go of the brake the second the gate drops, you'll get an early speed boost. Big jumps also have a way to gain speed (you need to prep with X, then release just before you land). You also start out with one 'anytime' boost.
I've only raced around 8 races so far, but the tracks seem solid. My guy is obviously low leveled, so I'm assuming once I have him stat'd up I'll be tearing up the course.
Visually, it's nothing to write home about. The gameplay is more arcadey than sim (I think motoGP is where you'd go for sim), though you can definitely crash into the sides of the course, etc. While the gameplay is nothing alike, I'd sort of put it on the same level as, say, Motorstorm RC --not AAA quality, but serviceable and enjoyable. It does feel higher quality than something like Asphalt Injection. Frame rate seems constant but I don't have an eye for that.
Oh, and the game has in game advertising for a brand name energy drink. Heck, you drink one to get your 'anytime' boost. It's probably the most annoying thing about the game, but it fades into the background pretty quickly.
If the idea of motorcrossing around in the (badly textured) mud on your vita sounds appealing and you're the type that can overlook flaws and adjust to a given game's racing style and quirks, you'll likely enjoy the game. Just don't expect great visuals or anything like that.