Miss-Take: The Curious Case of a Pianta's Intelligence

Professor Toadsworth's findings during his now infamous Isle Delfino holiday with Mario, Peach and several disposable slave Toads, explains why the Piantas thought Mario was the graffiti culprit despite arriving on the island after the crimes were commited. 

In regards to the unjust trial which denied Mario the right to defend himself in court, it is well known that Piantas are extremely racist towards Italian humanoids.