Does Mahabharata say somewhere that Duryodhana was a better mace fighter than Bhima? If so, was that why Bhima used unfair tactics in their climactic fight?
Does Mahabharata say somewhere that Duryodhana was a better mace fighter than Bhima?
Yes, it was said by lord Krishna when the last fight between Bhima and Duryodhana was going on that Bhima is mighty than Duryodhana but Duryodhana is better mace fighter than Bhima. It is mentioned in Mahabharata, Book 9: Shalya Parva: Section 58.
"'Vasudeva said, "The instruction received by them hath been equal. Bhima, however, is possessed of greater might, while the son of Dhritarashtra is possessed of greater skill and hath laboured more.
why Bhima used unfair tactics in their climactic fight?
Because Bhima had taken vow to break the thighs of Duryodhana when the later has insulted Draupadi. During the last fight between them lord Krishna reminded Bhima about this. It is mentioned in Mahabharata, Book 9: Shalya Parva: Section 60.
The accomplishment of one's vow is one's duty. Formerly Bhima had vowed in the midst of the assembly that he would in great battle break with his mace the thighs of Duryodhana. The great Rishi Maitreya also, O scorcher of foes, had formerly cursed Duryodhana, saying, 'Bhima will, with his mace, break thy thighs!' In consequence of all this, I do not see any fault in Bhima!