Yes, but characters don't need to either be exclusive to looking cool and being vacuous or being lame but with depth. The franchise has several that do both well. Grievous is hardly the first who imo doesn't, but I don't much care for ones like Boba Fett or Darth Maul either. They look cool... but that's about it.
I'm aware of what Star Wars' priorities are... but again, that doesn't preclude the ability of making your aesthetically "cool" characters actual characters instead of coming up with a design and then tacking on more too them down the line when you wish to market them further.
Is that not similar to the creation process of Nintendo's people like to gush about where they come up with the mechanics and the gameplay first (aka the important stuff), then worry about aesthetic design later? I just don't believe movies and video games are mediums where the aesthetic should be the first priority.
But what's even wrong with people liking a character for its aesthetics? I never here anyone complain whenever someone mentions R2-D2 or C3-PO as their favorite character.(People like them purely for athestic unless you want to argue that R2-D2 has such a rich backstory.) But as soon as someone mentions someone like Bobba, Grevious, or Maul people go onto this whiny predictable rant on how they don't have much depth as characters.
I mean, it's not like any of those characters play HUGE roles in the films when it comes to the depth of it. They usually just serve a single role to move the plot along.(Besides Maul who had reason to be developed more.) They could've easily just have been random general and dude who delivers Han Solo. I mean, that's really all they are in the movies, I don't see why it's so bad to make them cool even with a small purpose.
You only have so much time to fill in a movie dude, not every character is going to be even focused on especially in movies like Star Wars with so many. In the movie with could've easily gotten a character like some random neimoidian commander for Obi Wan to chase down instead, and while technically, it wouldn't change the plot at all and the character would have much depth in terms of the plot, no one would give a ****. Because they wouldn't have provided a unique villain even with such a small role. Sometimes in a movie you do need a character with just an intimidating/interesting design, especially when you only have so much time. YOU DONT HAVE TIME TO MAKE EVERY CHARACTER A FULL CHARACTER in a movie, at most you can do that in a book or a movie. People love grevious for the same reason people would love and remember something as Jabba the Hutt.(Who was also a secondary villain with little focus other than basically being a ganglord space pimp. I'd say he has more of a role similar to Jabba than Boba since Boba is hardly more than a background character. It's kind of like questioning why people love the T-Rex from any of the jurassic park movies. Grievous plays the role of a threat rather than a character, he's not suppose to have depth, if he had depth in Revenge of the Sith it'd make little since because outside of Kenobi's encounters with him, he as a character has no influence in the plot. Again, he's more like a force that one of the main, focused characters has to overcome.
Things such as the novels detailing him as a character, which shows that he clearly has potential as a full on character. Which is why his design is important, it actually plays into his backstory. But in the RoS, he wasn't suppose to be in an in-depth character, so complaining about that makes you look pretty ignorant.
Also, your point makes little sense, if you were talking about whole films, then yeah, but you're complaining about minor characters being aesthetically pleasing.(Besides Maul who has a lot of reason to have more depth than he does in the film.) And they have more depth that people do love anyway, it's just not presented in the movies.