If you don't want to be misunderstood, you should say "잘 모르겠어요" rather slowly 몰라요 in a rude accent could mean: Stop annoying me. As if I would care. Not a problem of mine. It's all your fault.

I don't think not only 모르겠어요 but also 몰라요 doesn't carry a negative nuance. but if you worry about that and say formally, how about add subject? "나는" 몰라요. "저는" 모르겠어요.