I remember those ships a while back - weren't those guys kinda the Borg of your universe? The unstoppable juggernauts nobody could beat, right? I remember them being very unconventional, and working the menace quite well.
However, in this environment they look more pretty than sinister....just something to think about. You might want to find another way for them to stand out as something creepy against an otherwise pretty tranquil setting.
Exactly, you remember them well. Any thoughts on how make them look more 'creepy' on an image where there is no weapons fired, or even other ships to threaten? In most established universes, this is mostly handled by the backstory of the ships, which the viewer automatically associates with it. Examples for me are Borg Cube and Executor images. You perceive them as menacing even is they are alone, because you 'know them'. No one 'knows' my original stuff yet.
Actually, the image was a result of experimentation with some techniques, and the long ongoing urge to do more original universe stuff, but story wise they would be quite harmless on planet images, becasue they do not attack planets.
I think the biggest problem with them is that they seem curved, almost a little too flowing, a little too gentle in their shapes. It's like someone trying to stick Starfleet curves onto a Borg Cube.
I think if you really want to do an "evil" ship or at least a "baddie" ship, you might want to consider the use of sharp angles in its construction and appearance. For example: Note the Imperial Taiidani Heavy Cruiser, the Borg Cube, and the Executor. There's some round shapes floating around in there (engine nozzles, guns, etc) but most of the angles on them are sharp, and very visually powerful. They all use a sense of solidness, of bulk, of blockiness that implies a juggernaut that will plow through anything you can throw at them. Your Triangulators have this going, but taking the subtle curves may really bring that vibe to the forefront here, and make it stick in people's heads that these are not a good thing to have approaching your planet.
Mmm there's an idea - Why not make the Triangulators more...triangular? If angularity really defines a ship's evilness in sci-fi, you might like trying a more obvious double tetrahedron shape, with the tetrahedrons attached to one another along one face. Like this <|> I'm not talking a drastic redesign. I'm just suggesting tweaking the angles and the existing shapes you've got now to play with some of these ideas.
Also, some details and greebles might be useful here too - some plating, some kind of detailing like the Cube or the Taiidani HC might help in the vibe too. They seem a little too smooth for my liking here, which just adds to your curvature issue that I mentioned above.
I know that was a lot to put in there, but I hope some part of this helps you with the design and the picture there.