Like any aspect of a story, a good villain needs three things: a beginning, a middle, and an end.
Beginning
The beginning doesn’t necessarily mean a backstory. It also means a motivation, a reason for opposing the hero, which can be explained by the backstory. It doesn’t need to be, though, or even be deep; depending on the level of the villain, it can be something simple. For example: the Wet Bandits from Home Alone. Their motivation is that they want to steal stuff for money. That’s all you need for them. We all understand wanting money.
For the Time Twins, their beginning is shaky at best. They’re elemental masters, two new ones which have never been brought up before with a new element that’s never been seen before (so now Water, Air and Time were unnecessary for Chen’s Anacondrai spell). Apparently all records of them were erased, which is why no one heard of them, even though this was never done for any other villain, and that one painting of them with the other elemental masters still exists.
Also they were defeated because they decided to shoot their powers at Wu and Garmadon. Even though their powers… don’t shoot.
As for their motivation: they became evil and wanted to take over the world, about as painfully generic a motivation as you can get. Because “their element is the strongest”, while Wu has the power of Creation. Okay, you can say that they’re arrogant, they believe themselves stronger, Wu hardly ever uses his powers, that’s fair. It’s still very generic, especially when compared to such villains as Nadakhan and Harumi, the two before and after them.
Granted, Ninjago has had such generic motivations with Clouse, Overlord, Garmadon, Chen. Many of them made up for it with their relationships to the main characters: Clouse and Chen hate Garmadon in particular, Garmadon loves Lloyd. Krux and Acronix kinda have a vendetta with Wu, and they have knowledge of Kai and Nya’s parents, but this doesn’t effect them, it effects Kai and Nya.
Wait! There’s one more. Krux hates modern technology and wants to get rid of it. And Acronix doesn’t. But then Acronix agrees to go back in time anyway, never voicing any complaints, so… I guess he just forgets this character trait.
Middle
The middle can be split into two parts: what the villains do, and how they do it (what their powers are, etc.).
So they’re elemental masters. That’s a good start, elemental masters are a well-established thing in Ninjago. It creates problems with the story, but as a concept, it works. But wait, they are different: there’s two of them. Wu says it’s “like the hands of a clock”, but a clock just represents time. There are three and four-handed clocks. Heck, the celestial clock in Stone Army is an example. It would make sense if their powers were inverse of each other, and half of them are (forward/reverse), but the other half are Pause/Slow? One is just a more powerful version of the other. It’s the only element to split like this; if twins are born to any other element, only one inherits the power. So how would the element be inherited from them? Would it just go to the first one to have kids?
So what do they do? Well, first they gather the time blades to get their powers back. That makes sense. Then they hatch an army of vermillion. More snakes that are related to the Serpentine but were never mentioned before, okay, whatever. Then they build a machine to go back in time. As I said, this goes against Acronix’s character, but okay, it’s cool. However, it’s also Time travel, something that is hard to handle. Ninjago doesn’t have the greatest track record of this (See “Wrong place, wrong Time”), but here, it’s handled fairly well, with them correcting the timeline and then drinking Mystake’s Tea to forget the events that weren’t supposed to happen. They also try travelling to the future, where no one knows them.
I can nitpick Ninjago’s power system to The departed realm and back, but their actual actions make sense. A decent middle, all things considered.
End
Ho boy. The end, where the villain gets defeated. And as we all know, the time twins… don’t have one.
At the end of their season, they are lost in time with Wu. Were this not Ninjago, they might have another season where they find Wu and fight the time twins again, but Ninjago has to have new villains each wave so they can have new sets and minifigs. That doesn’t mean they can’t return, though; we’ve seen returning villains such as Clouse, Pythor, Harumi. But then next season, Wu returns… and they don’t. And that’s the end! They were defeated off-screen.
Okay, so it’s not, but with Wu back, there was no indication that the TT would ever return or get an explanation as to what happened to them. This criticism is based on what we knew at the time.
But wait! Then there’s Golden Hour. First off, this was written by a fan, meaning that it’s entirely possible that the creators never planned to address K&A again. Second, it really answers nothing. They have several battles across time, the iron doom is broken more, and then they’re all still in the time vortex. Which Wu still somehow escapes from. I guess it could explain why they haven’t – they were separated from Wu, so he escaped without them. And at least they were “defeated” on-screen.
So they’re lost in time. It’s like being in Kryptarium prison, right? Nope. It’s a prison they could escape from at literally any time, and they still have their army and superweapon. In fact, Tommy has suggested that they might already have escaped and be out there somewhere.
Conclusion
A good villain needs a beginning, middle, and end. The Time Twins had a shaky beginning, a decent middle, and a non-existent end. To put it one way: they were just there. They show up out of nowhere, do some admittedly cool stuff, and then disappear back into nowhere. If you watch Season 7 with no context, it is fairly decent, and the end may leave you wanting more. With the context of the show, it just doesn’t fit, and you realize there is no more, the Time Twins just disappear forever.