Yes, Ann Coulter's tweet is offensive because its implication that a person only be allowed to vote if all four of their grandparents were born in the US is xenophobic. The United States of America was founded by immigrants and the constitution protects the rights of any natural born citizen, regardless of what their family lineage is.
Coulter is not a stupid woman, but she plays one on TV. She has long since realized that being shocking is more lucrative than being intelligent so that is the character she plays. This is a dangerous game. I blame the media who enable her with the fantasy that no one really takes her seriously even more than Coulter herself.
She's entitled to her beliefs. However, Coulter's 4 grandparents tweet was ignorant and offensive. It smacks of the "racial purity" rhetoric. Plus, by her logic, Trump himself and most of his offspring would be in-eligible to vote. She could just as well have said only white folks can vote. Remember we're the immigrants to this country. There were many native people living here before the Pilgrims arrived.
No, it was not offensive. I believe that Ann Coulter's pro-Trump tweet was not offensive. He was just trying to open his heart and let out whatever he thought had bothered him about Trump. By so doing, people will always judge you. But according to the tweet, he only said nothing but the truth.