The mistake here was not the illegible stamp. The mistake was the brazilian agent thinking that a Portuguese stamp means anything to anyone else other than other Portuguese (or Schengen) agents. Stamps are just record keeping system for each country's border agency, not some way to communicate with other countries. It's an old fashioned system based on paper and having you carry around the paperwork instead of an online system that is kept in their possession but that's all it is.

That makes the Greek agent's reaction more understandable. If that agency isn't concerned about the dates on the stamps (presumably because they know the online records have the information recorded) they don't care about anyone else being able to read the date.

Fwiw when I needed to document my travels I cross checked my records against the stamps in my passports and found several errors. There was at least one case where there was no stamp even though I normally would have had one. And there was one time when the stamp had the wrong date.