If I had to pick a name for this kind of slang/phrasing, I think it would be something like “superlative trunc”, as in a truncated superlative (biggest, smallest, oldest, richest, tastiest, etc).
I have a friend who suddenly realized they had a habit of blowing on very cold bites of food (like ice cream) as though it was something too hot to eat. They rationalized this as needing to do it for any food that was “too temperature.”
There's no mechanical/grammar reason but the first one is used sarcastically. When someone thinks something is a big deal/great/awesome and you say back "well that's certainly a thing" it's implying that the only thing you agree on is that that thing exists, not that it's great.
The other two are nonsense and some people just find that funny.
"That's certainly a thing" refers to phrases such as "You definitely said words." It expresses that one wants to acknowledge that something happened which demands commentary, but the commentary is self-evident, and thus sarcastically skipped.
I think Tumblr's particular grammatical quirks are fascinating. I'm pretty sure it's a result of people replying to posts in the tags, which have some limitations (obviously no commas, exclamation points change the order of tags, no quotes, certain themes changing capitalization, etc). Over time, it's just become the way people talk on there.
You see someone post a drawing, and it's really not good.
"That's certainly one of the drawings of all time"
The expectation for the phrase would be "that's one of the best drawings of all time" or "one of the worse drawings of all time" but removing the adjective to break the sentence creates this ironic feeling of stating something is underwhelming without directly saying so.