A prayer is a conversation with God. What are you telling God? That Mr Mandela should be given some physical relief from his illness simply because he is a human being in pain, or that he deserves physical relief from his illness because he is a great guy who deserves it? The OP has a point. No one is good except God alone, said Jesus. Telling God that X is good because you think so could be telling God something with which he doesn't agree. How confident are you that God agrees that you are speaking the truth when you utter your words of prayer?
The OP's point may be that one can pray for what one wishes in charity as long as the prayer isn't unnecessarily embellished. Truth is owned by God. An inaccurate embellishment is an invasion of his property rights over the truth. It's saying that something is when it isn't. While it's OK to ask God to provide an old and sick man with a little bit of pain relief as a matter of charity and to leave it at that, it may be overreaching and vain to seek to justify with other reasons as to why God should provide the pain relief.
Nevertheless, bearing in mind Pope Francis' recent admonition against calumny, is Mr Mandela an ogre? According to what we are told by news and commentary on the public record, it seems he developed a quiet peace during his long spell in prison, cooperated sincerely with President de Klerk to effect a peaceful transition to majority government, stood for and won election as president to bed down the crucial birth years of the new republic and, unlike others who developed a taste for power, refused to succeed himself in office when he could easily have become a wannabe president-for-life like Robert Mugabe.
When you turn up at the Pearly Gates, who would you want God to regard as the real you, the reformed person you became a few bits of time before you died or the uninhibited hellraiser you used to be for thirty years before your repentance and conversion? Like the labourers who were reckoned by the owner of the vineyard to be due a full day's wages because they chose to keep up their hope for employment patiently for the whole day and to accept the owner's invitation for last minute employment without knowing what they would be paid, perhaps the last five minutes, so to speak, of Mr Mandela's life is the one we should choose without violating the truth to remember him by, rather than by his early years as an active terrorist and, dare we say even, the guy who ditched his plain first wife for a sexier second one.
But while, in charity and to avoid any risk of calumny, we might choose to remember Mr Mandela in his later form, it would be presumptuous to tell God that that is how God should think of him. God has responsibilities in justice to all manner of people who may have been affected what what Mr Mandela said and did throughout the whole of his life, his spurned first wife, perhaps, and those who died in various terrorist activities, and so we should let God think of him as God wants to think of him and phrase our prayers modestly to ask what we need to ask and just that.