When Uncle Axel describes Tribulation as nothing more than a "word... A rusted mirror, reflecting nothing," he is explaining how the way in which the powers that be in this dystopian world treat Tribulation as if it were a rusted mirror. That is, they only see what they want to see and refuse to allow reality or doubts to intrude into their thinking. Uncle Axel realises that Tribulation, which young David is asking about, is not exactly what the preachers of their time state that it is, and if the preachers could see Tribulation as more of a clear mirror, they would be forced to challenge their own ideas and thinking about it to a great extent. Consider what he goes on to say:

It'd do the preachers good to see it for themselves. They'd not understand, but they might begin to think.

If the preachers could actually see into this mirror and see Tribulation and what they have made it, they would ask themselves some very hard questions about their ancestors and whether they were right or not. What characterises the preachers of the present, according to Uncle Axel, is blindness, in that they are not able to see beyond their own opinions and beliefs, even when they are clearly erroneous. This is why he links the Tribulation to a rusted mirror.