I mean more that Arceus's appearance makes Arceus feel less powerful/awe-inspiring than a creator god should be. I mean more that part of what makes the Original Dragon intriguing is the mystery, compared to catchable Arceus honestly doing more to hurt the image than improve it.
I don't really feel like Arceus had much wind to begin with. I went on a rant in Unpopular Opinions, but I just don't like the concept of that Mon actually being part of the roster.
I never really thought this, considering gods throughout mythology always take on basic human or animal form, not really anything unusual or special. The mystical and awe-inspiring things about them were what they could
do, and their history, not necessarily appearance. Cronus is just a man with a sickle, not really a big deal, but when you see a painting of him among stormy clouds devouring his child, then you get a sense of what this guy's all about. When you go the Hall of Origin which is huge, completely empty, and just dripping with atmosphere, you get an idea of just how big of a deal Arceus is. When you access the HGSS event and there's an Arceus literally creating another god-level Pokemon, you get an idea of how powerful he is. It's not really about the design of Arceus in a vacuum, it's about images like this:
Although personally I'd say Arceus has a great design, that gives him an impression of being very powerful without having to be stupidly overdesigned to get a point across. The fact that we're not quite sure what kind of animal he is (he seems to be in the horse family but really it's not clear) and the fact that he is an animal at all makes him far less relatable and way more mysterious.
As for being able to catch Arceus making him seem weaker, eh I never got that idea either due to gameplay and story segregation. Because it's the same for almost every legendary Pokemon that are gods, like Groudon, Kyogre, Dialga, or Palkia, even if Arceus is on a much higher scale. Even then, Arceus does outclass almost every Pokemon and its versatility is second to none thanks to the plates. Besides, in the Sinjoh Ruins, Cynthia says that Arceus chose you and pretty much you can only command it because it deems you worthy and allows you to, so perhaps it's limiting the amount of power it allows any one person to wield.