In 2012, Obama was subpoenaed in the case Farrar, Lax, Judy, Malaren, and Roth v. Obama. However after a failed attempt to quash the subpoena on the basis that a sitting president cannot be subpoenaed (which Judge Malihi rejected and ordered the President to attend court) the White House decided to simply ignore the subpoena and neither Obama nor his lawyer attended the hearing. Was it legal for Obama to ignore this subpoena? (I recall that Nixon was also subpoenaed and the Supreme Court found that he had to comply.)
No, it is not legal for the president or anyone else to ignore a lawful subpoena.
Under United States v. Nixon, 418 U.S. 683 (1974), the president is free to invoke executive privilege in an effort to quash the subpoena. But he may not simply ignore it.
It is not clear, though, whether the Georgia judge (who was not a real judge, by the way, but rather an administrative law judge) in this case actually had jurisdiction to compel Obama to appear.