This would be a bit odd if it was in the 1500's or even in the 1600's, I've seen plenty of records showing Spanish people as poor, situations are many, and by the 1700's societies in Mexico where no necessarily driven by race as we may think, for example Peninsulares had (usually) more rights than Spanish Criollos, been "white" wasn't enough any more, one of the reasons for the Independence.

Now the "poor" condition could have a myriad of backgrounds, for example not all kids in wealthy families "make it", nor all had inheritances, we find often siblings in a total opposite situations. Some times the parents would inherit or give everything they had to their kids passing away in "poverty", some loose their wealth, and some where never rich at all, many came to the continent between the mid 1600's and 1800's as a regular workers, with no much in their homeland and lived as a laborers for all their life, and the list can keep going.

Now the interesting part of that document is the fact that the husband is address as "Don", but even there we know that since the days before Cervantes there where plenty of "Hidalgos pobres", this is, not even a social title would secure wealth.

Note that in a few instances, the inheritances and conditions where "hidden" as to not provide the church with any fee/donation, something similar goes with the famous lack of signatures, many people did know how to sign but refuse to "accept" it, giving away signatures was a tricky situation.