It’s conflating two trademarks so that people who own neither can profit. And again, if that isn’t copyright violation, I don’t know what is.
Trademarks and copyrights are different things. This is a copyright case and a copyright ruling.
Copyrights protect an expression of an idea, but not an idea (or a style) itself. You cannot copyright the drawing or prose style of a Dr. Seuss book.
You cannot copyright a character in a sci-fi franchise. You can copyright a specific illustration, you can copyright a specific textual work, you can copyright a screenplay or a television show.
Combining ideas from two other sources and blending them together with your own ideas can result in a new creative work that is copyrightable. Let’s not overlook that this book does include a new story, new prose, and new illustrations that, while they have features in common with some previous works, are indeed new and novel.
Let’s not forget that this kind of mashup was a longtime staple in Hollywood. “High concept” boiled down to mixing key ideas from popular movies or shows in order to create something new. Hell, Star Trek creator Roddenberry pitched the original series as “Wagon Train to the stars.” He didn’t need to get permission from NBC because he borrowed the idea of a group of characters journeying into a frontier from their popular western series.
In a legal sense, a trademark is a recognizable branding elements that consumers are supposed to be able to rely on to know the origin of a product. You could have a trademark in an image of Kirk or Spock or the Enterprise, and put it on the packaging so that your customers know this is the real deal, official licensed toys. The Cat in the Hat logo is probably trademarked by the Dr. Seuss estate, and only printed on their official publications. The key to a trademark suit is that it has to be identical or similar enough as to confuse potential customers as to the source of the goods it’s on. I doubt that there is a trademark claim to be made here because the Seuss Estate or Paramount (or CBS or whoever the hell owns the Star Trek rights now) would have filed a trademark case in addition to or instead of this copyright suit.
In a conventional sense, a trademark can be something more general, like a style. You can say there’s a trademark style to Dr. Seuss prose and imagery, but that would have no bearing on a copyright case, nor is it likely to be relevant in a trademark lawsuit.