Edward basically has very limited presence, but what you say is also true: we mostly don't see him through the eyes of Elinor. Marianne tends to express her opinions more energetically than Elinor. Logically, I think she made a disproportionately huge deal out of his reading Cowper the wrong way. But if that and being quiet is all we know about him, even if our source is Marianne, then it's perhaps no wonder he comes off as nice but colorless.