I actually think Jet Hammer is not nearly as useless as many people make it out to be. Against both poor and solid players, I've found the Jet Hammer to be really good in primarily edge guarding situations. I'd reckon it's one of my more solid ways of getting a kill, even, and I wouldn't call myself shabby at Dedede.
In my experience, it's all about the set-up. Obviously, it's not a great move to use in just regular combination. At low percentages, you're just not gonna get careless reactions from people that would slip them up against a Jet Hammer, and the knockock/damage isn't worth it in comparison to Dedede's other standard attacks. However, when you've limited enemy options, it can be a solid kill from anywhere in the stage. The ability to walk, turn, and jump with the darn thing is it's major highlight, and you gotta know how to use that.
The typical set-up is getting them off the edge (not very hard), using an up-tilted Gordo, and preparing the Jet Hammer. Now, all of this is obviously based on the enemy character, but you've really narrowed down their options. Trying to hit the gordo away without a projectile is usually a bad trade, since you only have a handful of air attacks to use in that situation. Waiting for the gordo to roll off risks getting hit by it when the invincibility frames wear off, and that can result in a quick death. What you've mostly got left are options not very favorable - standing back up from the ledge allows Dedede to just walk forward and kill; rolling back simply requires Dedede to turn around and, again, kill; jumping is a bit safer, but if you catch it soon enough, you can also jump and chase, getting the kill mid-air; or, if unaware of Jet Hammer's reach, you simply hold on to the ledge and stall, where Dedede will happily finish the encounter.
Overall, it provides a situation that the enemy doesn't want to be in, and forces them to act immediately - the longer they wait to make a decision, the more power that Jet Hammer will have, and Dedede usually has little to risk. Add in the shield breaking factor of this move, and it's even more nerve racking for the enemy to deal with.
I think most people dismiss Jet Hammer simply because it doesn't have a huge versatile use, but in its places where it excels, it excels big time and can decide games. Just like Gordos, you just have to get out of the habit of abusing the attack.
And of course the other big use is against people abusing recovery specials, especially teleports like Palutena and Zelda, where you can easily predict where they're gonna be in a particular area, and waddle right up to them for a powerful hit. All in all, you shouldn't write this move off so quickly.
