Yes, that's the simplest way to make the encoded tweets look natural, i.e., truly steganographic, but it's a fine balancing act between that and not making the corpus obvious.

Is it a big deal if it's obvious that the corpus is of tweets? There's many many ways to build one and order the tweets, right?

The content of the corpus doesn't matter, so the answer to your first question is no, and yes, if you're clever in terms of how you've composed the corpus so that no one could re-create it easily, then it's ok (relatively speaking, of course).