Is saying, "So-and-so is not the best painter," considered backbiting if the sole purpose is to inform?

If there is no overriding legitimate benefit in reporting a specific statement, such as advising someone, then it should not be uttered. It is feared that it might fall under prohibited backbiting, which is mentioning your brother in a way he dislikes. The Salaf (pious predecessors) were very cautious about backbiting, to the extent that Ibn Sirin sought Allah's forgiveness when he said about a doctor that he was "more skilled than others." The safety of one's religion lies in avoiding such talk unless there is a legitimate reason, and in acting upon the Prophet's (peace and blessings be upon him) advice: "Whoever believes in Allah and the Last Day, let him speak good or remain silent."