The original Nook Color and original Fire are both 1024 x 600 which is 169 ppi, and both are easy to read on without eyestrain. At least for me they are. The iPad1 and iPad2 have 132 ppi, but they do cause me to have eyestrain. So somewhere between 132 ppi and 169 ppi is the resolution at which my eyestrain will go away.
If the leaked photos are accurate it looks like the Mini will have a 16 x 9 screen much like the smaller iPhone5. So if they keep the width pixel count at 768 to match the iPad2, then the dimensions would be around 1365 x 768, which would have a 200 ppi resolution. Not as good as my iPad3 with 268 ppi, but more than adequate for reading over long durations. (BTW, the Nexus 7 and Fire HD both have 216 ppi.)
Unfortunately though I think the iPad Mini looks fairly big compared to a Nexus 7, and not much smaller than my iPad3. That coupled with what will surely be a high price tag of $350 or above will make it look not too interesting to me.