Goofy appearance (never dug the dark magic reindeer monkey), no plot relevance despite appearing in the intro cinematic and a pretty good boss fight that From had so little faith in, they gave players an item to just negate his magic attacks outright. There's a reason the DLC's named after Artorias instead.
Maybe it's goofy for you because the boss arena is too dark?
Also, we could argue that Manus isn't THE Furtive Pygmy mentioned in the intro cinematic, from how Marvelous Chester didn't specify if it's *a* primeval man or *the* primeval man. For all we know Manus could be the child, the grandchild, or Xth generation not too far from the Furtive Pygmy. And in the chance that Manus *is* THE Furtive Pygmy, then the intro made it clear that he's
"...so easily forgotten." and stopped there.
Regarding the Silver Pendant, I'd argue it's there for the same reason they give you the Storm Ruler when dealing with the Storm King in Demon's Souls/Yhorm the Giant in Dark Souls 3. Since I haven't played DeS, and I know Yhorm can be beaten just fine without the Storm Ruler, would you say From have little faith in the Storm King/Yhorm the Giant when they put in the Storm Ruler?
Also, from the lore perspective, think of the Silver Pendant working in the same way as the Covenant of Artorias.
You may think that this is just some kind of lorefaggotry, but like I said, I have no impression that the narrative aspect of Dark Souls 1 being as much of an afterthought as Dark Souls 2&3. It's clear that they put in a lot of effort into crafting the world of Dark Souls 1, and it shows just as much in their narrative aspect.
Finally, the DLC being named after Artorias was most probably for marketing purpose, since Artorias has been hinted at quite clearly through the base game, whereas Manus, let alone The Furtive Pygmy, are mostly nonexistent with the exception of comments made by Kaathe (and even he was still mostly being vague). Would you complain that the final boss for the Crown of the Old Iron King DLC is the Fume Knight, or that the final boss for the Crown of the Sunken King is Elana/Sinh?