No it isn't. The set {theist, atheist} is covering, so you have to be one or the other. Avoiding the question is fine though. It's a topic that often leads to pointless arguing. (Often of semantics, like I'm doing now ;)
The definition of agnostic is completely pointless. If someone was a theist or atheist, that would be what they would call themselves. If you go to the trouble of calling yourself an agnostic, you mean to say "I don't know one way or the other."
Not pointless, just answering a different question. An agnostic atheist would say "I don't know one way or the other, but I don't believe in a god". An agnostic Christian would say "I don't know one way or the other, but I believe in Jesus". Versus gnostic atheist you would say "I know for certain that exactly zero gods exist", or gnostic Deist who would say "I know for certain that one or more gods exist"
Hold up: according to Google (and my mind), an agnostic is "a person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena; a person who claims neither faith nor disbelief in God." As far as I am aware, it isn't a qualifier. It's a section unto itself, like atheist or theist.
It's one of those terms where the definition is very muddied. Certainly a lot of people use the term that way. That definition comes from an incorrect definition of atheist, which is incorrectly labelled as disbelief, when it is actually lack of belief. While similar, they are distinct. Take the case of a 2 day old baby. It isn't capable of comprehending the idea of a god in order to have disbelief. You can safely say that it lacks belief in a god though.