Cruz announced at a rally on Wednesday in Indianapolis that the former Chief Executive Officer of Hewlett-Packard would join his ticket.
But the unveiling faced scorn on social media after she started to sing while addressing the crowd.
"I know two girls that I just adore," crooned Fiorina, in reference to Cruz's two daughters, Caroline and Catherine.
"I'm so happy I can see them more. 'Cuz we travel on the bus all day; we get to play."
Twitter reacted with bewilderment, mocking Fiorina over her singing.
"Carly Fiorina started singing at a rally where she's named VP by a candidate who ain't getting the nod," tweeted writer and Ebony columnist Michael Arceneaux (@youngsinick). "This election is hilarious."
"Fiorina is singing and it's extremely creepy," tweeted Donie O'Sullivan (@donie), the politics lead with news agency Storyful.
"You know this was a joke but then she literally went and sang at the event," tweeted joseito (@go_oat).
After suffering a series of crushing losses to Trump in nominating contests on Tuesday, Cruz praised Fiorina as a principled fighter for conservative values who knew how to create jobs and would be a valuable ally on the campaign trail.
