Besides Dragon Quest, which is a very large game series, you picked 3 games that are recent releases compared to Touhou which is a long running video game series.
Here's something that might be a more meaningful comparison, using multiple characters who have been highly requested in the Smash community.
As you can see, Touhou Project isn't that different from some big name series. (Btw, if you change Terry to Fatal Fury or King of Fighters, it still keeps it within the same area as the others.)
Not that Google Trends is the best or perfect way to indicate popularity, of course. It only indicates how many times it was searched recently.
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By the way -- just leaving brief thoughts.
One reason why Touhou doesn't seem as big to many in the Smash community is because, just like with other PC game series, there has always been a rift between the PC gaming base and console gaming base. Just think of it -- you don't hear support for Jim Raynor or Arthas for Smash, despite being huge game icons from VERY huge and successful games with a big legacy. We only heard discussion about Tracer recently because of the Overwatch Switch port rumor. I wonder how many people here don't even know who Raynor is, or even Arthas? It doesn't mean they're not popular. Heavy is the only PC based character with a good amount of support in the Smash community that I can think of, but he still is requested way less than he is for how popular TF2 is, compared to other characters like Shantae. The reason again is because of different fanbases, and how PC gamers naturally don't overlap as well as console gamers do with each other, for obvious reasons.
Different communities is a thing. Things like # of subscribers on Reddit, etc., are not 1:1 representations of how popular someone is. Sites like Reddit for example, are heavily male dominated. Touhou as a game series has much more female fans than the average game series, and in my personal experience with many of my female gamer friends, they simply don't use the same platforms that male gamers do, or they spend much less time on them. They don't like arguing online with random people but instead just talking in their own friend circles. There are many factors that can affect perception of how popular something is, and there will never be 1 perfect metric to measure it accurately.
The big thing holding Reimu back is not her being "unpopular" in the West. She's definitely popular and is known among many communities, not just gaming, and she has many female and male fans, casual and hardcore. The thing holding her back the most is that she's from a PC gaming series with low accessibility. You can't officially buy and play the game on PC until recently, with Steam releases for a couple of the newer games, but they are not even translated to English. And we're only starting to get some games on the Switch now.
Smash DLC is about having big game icons join, but I think an implicit understanding with that is that the character must be associated with Nintendo to a degree, or at least "fit" the aesthetic easily in some way. They aren't going to add Arthas or some Dota or League character, for example.