Small sample of 2642 ores mined using bombs only gave 38 Miner's Troves, for a drop rate of ~1.44% (~1.50% if assuming only basic ores drop MT).
When opening Miner's Troves, Diggsly drops on a separate roll from the other 5 drops (i.e. if you get a Diggsly, you will get it on top of the ores or diamond).