As onomatopoeia means words that sound like what they mean, is there a word which means words that sound contrary to what they mean? Pulchritude is an example of such a word.
You're shifting the definition here. "Onomatopoeia" means creating words that sound like the thing or the action that they describe. To the best of my knowledge, this is only used in the literal sense, e.g. we talk about the "buzzing" of a bee to describe a sound that at least resembles the sound "buzz". But in the example you give, you're talking about a subjective evaluation of the idea that a sound brings to your mind. That is, I have never heard anyone say that, for example, "philosophy" is a case of onomatopoeia because the word "philosophy" sounds long and impressive and has a tone that brings deep thought to his mind. That's just not what onomatopoeia means.
The vast majority of words in English are not onomatopoeic. The word "zebra" sounds nothing like the sound made by a zebra; the phrase "internal combusion engine" sounds nothing like the sound made by such an engine; the word "surprise" sounds nothing like the noises made by people who are surprised; etc etc. I guess you could call such words "non-onomatopoeic", I don't know of any specific word. I'm not exactly sure what an "opposite" sound would be. Perhaps you could say that "boss" is a very soft word for creatures who can often be shrill, or that "politician" sounds rather stacato and active for creatures whose talk is usually pretty dull and monotonous. :-)
Although your examples were not the right ones, according to your description, you are probably talking about one of the following:
Phantonym: An informal term for a word that looks as if it means one thing but actually means quite another. For example, unisex.
Antagonym: word that can mean the opposite of itself. Antagonym are also known as contranyms or autoantonyms. For example, "To overlook" can mean "to inspect" or "to fail to notice."
According to Wikipedia:
"A phantonym is a word that sounds to mean one thing, but in fact means another."