“Every candidate has their fringe followers. I highly doubt the Cruz campaign created this sign.”
Likely to be quite true. However, Cruz did smear Trump with the Duke and KKK accusations, so this incident could end up being seen, rightly or wrongly, as another intentional or unintentional continuation of the same unethical campaigning and dirty tricks. If the Cruz campaigners had nothing to do with it, the consequences may prove to be ironic.
He has been pushing the KKK thing pretty hard lately. I would not put much past Cruz.