Mandarin is good. In fact, I think he is severely underused, underestimated, and limited by writers into not using his full potential.
But the 10 rings vs Dr. Doom (complete with armor and magic) is not enough to beat him every single time. Doom is simply too smart and too versatile and in all honesty some of the rings are simply too one dimensional (ice and flame for example. It is like assuming that the Blizzard and Pyro would be a threat.) Used in conjunction, his powers are extremely formidable, especially the higher end ones (his matter re-arrangers, using his white-light one for different purposes, the Disintegration Beam, ect ect). For this reason I give him 4 wins out of ten fights.
And I give it to Doom 6/10 times because of the versatility of his magic, and the strength of his armor's defenses/offensives.
Does he though? Iron Man-lvl tech + at least high meta lvl magic = something that I would think could outstrip Mandarin in nearly every department. I know more about Doom than Mandarin, so I'll concede some ignorance on the subject, but I'm surprised to hear this opinion.
Doom easily he had beaten better guys then the Mandarin . Talk to me again when the Mandarin becomes more then an iron man villain and becomes someone that even Thanos respects.
That's with prep though. Thanos would eat Doom for breakfast in a "Ready? GO!" scenario. In such a fight, this is a good matchup, though I tend to agree with you on Doom.
The Mandarin is incredibly tough but even he isn't the walking plot device that Doom is. Doom is a master of science and tech to rival Reed Richards and a master of sorcery too. He's also been shown to be a badass physically without his armor. Mandarin is a superhuman martial artist, plus the rings and his tech but he's not quite on Doom's level. He'd need the deck stacked to win.