Hot Shots Tennis is really only disappointing in the face of how reliably enjoyable the Hot Shots Golf franchise has been.
The magic of the Hot Shots Golf games lies, among other things, in the fact that the intricacies of an extremely complex game are coated with a candy veneer. The cartoony characters don't truly represent how deep the game is and as such, it's both deceptively difficult (though accessible to the casual player), and a startlingly realistic portrayal of the game of golf. There are a number of things to keep in mind, and the struggle is in charting a plan based on a variety of factors coupled with the difficulty of execution at the point of action.
Tennis, on the other hand, is more a game of action alone, where aside from court material, environmental factors don't affect each play as in golf. Where golf is a game of planning and execution, tennis is a game of rhythm and stamina. To this point, every tennis video game I can think of (at least those that actually stick close to reality, which excludes the Mario tennis games, for example) has been fairly simple. There might various buttons for various types of shots, but I don't believe there have been any that approach the simulation nature of the Madden football, Winning Eleven soccer, or Tiger Woods golf titles.
Hot Shots Tennis sticks to this mold as well, and therein lies the problem. As previously noted, part of what makes the Hot Shots Golf franchise so fun is the way that its looks do not indicate its depth. That, unfortunately, is not the case with Hot Shots Tennis, and the impression the game makes suffers for it.
Although I'm a fan of character customization/creation when I see it, I don't normally miss it too much when it's not there. However, the dearth of such options in Hot Shots Tennis is noticed, primarily because part of the fun of Hot Shots Golf lies in giving your character upgraded equipment along with new clothes and accessories. Those performance upgrades aren't present in Hot Shots Tennis, and as such, you can't feasibly stick with (and grow attached to) the same character through the course of the game. Given that much of the Hot Shots aesthetic has to do with personality, this is an issue.
What makes Hot Shots Tennis interesting from a business standpoint is that it didn't come out for the PlayStation 3. Certainly, an argument could be made that the PlayStation 2 is not yet dead, and that there's no reason that quality titles should not continue to come out for it. But ever since the release of the PS3, it seems that the best titles released for the PS2 have been sequels to games that were already well-regarded (God of War 2, for example). In this case, what we have is an attempt at a new franchise using a familiar, time-tested format. It seems an odd choice to not launch this new endeavor on the newest possible system, considering that the Hot Shots games have been exclusive to Sony since the very first title.
I think that what all this comes to is that Hot Shots Tennis is really only disappointing in the face of how reliably enjoyable the Hot Shots Golf franchise has been. It just doesn't have the same spark. In some sense, it seems unfair to be discussing nearly every aspect of the game by comparing it to previous games by the same developer; on the other hand, given that the game is clearly composed of the same design elements, and its title would have us believe it's part of the same universe as Hot Shots Golf, it's difficult to discuss this newcomer to the Clap Hanz roster without making references and comparisons to past successes.
Hot Shots Tennis isn't a bad game by any means. The problem is that it isn't as fun or accomplished as the golf games to which it will inevitably be compared. Many reviewers have made comments about how this game isn't as fun as its golf counterparts. I certainly agree with that sentiment, but I'm not convinced it could be. This may be spoken through the lens of someone that prefers golf to tennis, both as a player and spectator, but I'm not sure I believe that, given the strengths of the Hot Shots Golf franchise, there aren't sports that are better suited to the Hot Shots treatment. I, for one, would love to see Hot Shots Baseball, or even Hot Shots Bowling. That said, it also seems very possible that Clap Hanz will take lessons learned from the reception to Hot Shots Tennis, and address these issues for the sequel.