The concept you're looking for is the https://en.wikipedia.org/wiki/Genetic_fallacy, a broader form of ad hominem.

Also, they didn't say it's bad because it's from reddit, the statement could be interpreted as saying that it's bad, and also that it's from reddit. Imagine http://laurencetennant.com/bonds/adhominem.html applied to the genetic fallacy, and you'll see what I mean.