Most of the roster are characters popular WORLDWIDE.
Also, who said Dragon Quest's inclusion would be to "spead popularity in the West"? Dragon Quest has impressive enough credentials to enter Smash on its own merits, which makes it far from a Corrin situation. Dragon Quest III in particular is probably the most popular Famicom game outside of the two Mario titles (1 and 3), and the Famicom in general is still among the most beloved game consoles of all time.
Also, if you are a fan of Banjo-Kazooie, Steve, or Doom Guy (particularly BK or DG), I hope you realize that most of their popularity is Western, and they could very well be seen as "shill" picks by the Japanese fanbase to cater to Western audiences. The fact is that for some characters important to Nintendo or gaming, they simply have lopsided popularity in certain regions. Ridley was among the Top 3 most wanted characters for Smash Bros. for the last 10+ years, yet there was not much demand for him among Japanese fans. The same goes for Little Mac. Punch-Out is considered to be among the most popular and iconic NES games of all time, and the Wii game was a major hit, yet among the Japanese audience, those games are considered to be very niche.
You are very much ignoring the legacy and impact that Dragon Quest has had on the industry. Your narrative that a DQ representative is simply "another Corrin" is profoundly misinformed statement. Dragon Quest has had DECADES of multi-million selling installments, is something the gaming public as a whole knows in Japan, not just the "core".
The narrative that "Dragon Quest is a shill pick" is peak "core bubble" nonsense.