Can I sign legal documents with a smiley face?
Yes, that is lawful. A person's signature does not necessarily have to include the person's name or initials. What matters is that the signature reliably and unequivocally identifies the person who produces it, which apparently you have been able to prove by showing your driver's license.
The Black's Law Dictionary (4th Edition) states in its entry for signature that "whatever mark, symbol, or device one may choose to employ as representative of himself is sufficient". It directs to the entry for sign, which likewise speaks in terms of "any mark, as upon a document, in token of knowledge, approval, acceptance or obligation". Accordingly, your signature qualifies as mark or symbol that fits these purposes.
Your history of signing other legally binding documents that way further reinforces the authenticity of your signature.
If it is legal, is it a bad idea?
It is a bad idea to the extent (if any) that (1) others can easily forge your signature (notwithstanding that forgery or identity theft might be proved circumstantially); and (2) verifying your identity may cause hassle or annoyance to you and/or third parties. But this paragraph obviously is applicable to any and all signatures, not just those which at first glance may seem to be a joke.