Ms Coxhead was inspired by people leaving messages on subway walls in New York after the US election.
She said the response - which has included "lots of bees" - has been "incredible and very comforting".
"I thought it was such a wonderful idea, because it helps those writing the message as well as those reading them.
"And I think we can probably all agree that [the wall in Piccadilly Gardens] is not the most attractive feature of Manchester."
'Feeling of love'
Shannon Smillie, who was working at the Manchester Arena on the night of the attack, saw the wall on her way to the memorial at St Ann's Square.
"I didn't know it was there but seeing it made me feel happy but also very emotional," she said.
"It just showed how such a beautiful thing can come out of a tragedy. And that's what I felt from it, an overwhelming feeling of love and comfort from people I didn't know."
Positive messages have also been posted on the outside of a mosque in Oldham whose door was set on fire just hours after the Manchester attack.
People from across the community responded by covering the Jamia Qasmia Zahidia Islamic Centre with dozens of colourful messages of support.
Morgan Harper-Jones, from the group LoveBomb which organised the response, said they wanted to "spread a basic message of love and solidarity" for Muslims in Greater Manchester.
A suspect was seen on CCTV setting a bottle of fuel alight outside the mosque but no arrests have been made in connection with the fire, police said.