Well, the bar relief and the idol, both found in "Call of Cthulhu" are each symmetrical representations of Cthulhu, although it could be argued that they were filtered through human minds, thus taking on shapes and proportions that exist in the deepest levels of human consciousness. Although that having been said, I prefer to think of them as much more real-life incarnations.
I don't remember anything from that story about the bas-relief and idol being symmetrical.
I think the fact that its symmetry, or lack thereof, is what confirms those objects as being symmetrical in design. And by this I mean that humans tend to only make mention of something that needs explanation. For instance, if a black man and a white man are telling the same story about a pair of guys, each would default to differentiating the other by color, since they think in terms of their own color being the default or normal state. One might say that "the black guy jumped" whereas the other might say, "the white guy had the chicken". By the same token, when Lovecraft is describing the bar-relief and the idol, at no point does he make mention of the forms being asymmetrical, only that they are otherworldly and alien. But when he describes R'lyeh, he goes to great lengths to talk about the strange angles and mismatched qualities of the symmetry. He differentiates to make the point that it is a different kind of geometry and by pointing it out, draws our attention to it. Notice he never talks about the weight of the bar-relief. One would have to assume that by calling it clay, it was logical to think that our minds would make the connections to the known quantity of clay, and thus not have to define it to us.
I think the whole thin is a case of confirmation by omission.