Wario as a parallel to shadow is something I never really thought about before. Both are essentially "evil versions" of the protagonist, but while shadow is a "cool", angsty brooding villain, Wario is a wacky idiot defined by his greed and strange quirks.
COme to think of it, it does make sense. Sonic is defined by his flippant, devil-may-care attitude and freindly demeaonor, so shadow is self-serious and sad. Meanwhile, mario is somewhat basic and generic, bareley even a "character" in his own right. What little characteristics he does have are essenitially common to most "hero" characters. Thus, his counterpart is brimming with personality, and is primarily motivated by self-intrest.