Am I (native Swedish speaker, so perhaps ignorant of secondary connotations here) missing something that should be obvious? Is mullvad inappropriate to some readers or is it just an odd name?
No, I don't think you are missing anything. As an English speaking American, it just strikes me as a strange name that I wouldn't immediately associate with a search engine. Note for example that the parent spelled it wrong despite that being the focus of his question. But there is no second level of meaning or innuendo that I'm seeing.