Mother of Mercy is possibly the most underrated Maiden song of the last 20 yrs.
I'm afraid this is Bruce's fault. The following explanation is a tad convoluted but please bear with me:
Whenever Bruce is trying to conserve his voice in live settings, he sings them in a particular "voice" in a particular range. An obvious example of this is when he sings "in the name of God, my father, I fly" in Flight of Icarus. Instead of singing it at the studio pitch (E5 I think?), he just sort of shouts it in the fourth octave range. This is pretty much how he sounds all over Live After Death and why I don't hold the album in esteem on Bruce's end as it comes across as strained on a good day. The chorus of Mother of Mercy is sung in this style, with the weird shouty strained vocals.
Further, I think most fans of Bruce's are drawn to his interesting phrasing choices. It makes him sound passionate even when he's singing something as dumb as "Alexander the Great... he died of fever in Babylon." But in Mother of Mercy he sounds so preoccupied with being in that weird ass range that he's not even thinking about what he's singing. It's a very straight take with little panache.
The song is also faulted ever so slightly by one of Steve's lyrical weaknesses. He has a tendency to overuse filler words when he needs to fill out syllables in a line ("just" is the usual offending word). Steve does this so frequently I never need to look at the writing credits of a song to know he's on there. The line "I'll die a lonely death, of that I'm certain of" is a particular abomination though because not only did Steve bust out one of his trusted filler words, but he did it in such a way that it's grammatically redundant. The line should either be "of that I'm certain," "I'm certain of that," or "that I'm certain of" (least grammatically correct). Songs don't need to be grammatically correct obviously, but that one just grinds my gears (I'm probably the only one though), especially knowing Steve's history of filler words.
All that being said, it's a fantastic song, and like many Maiden songs, without the chorus the song is a 9/10 for me.