There's a lot of problems with this clip. It's a comic beat, possibly the cold opening of the episode (unrelated to Mariner and Boimler becoming envoys), but outside of the jokes, it doesn't really fit the Star Trek universe. This is a parody clip, and I am really hoping that the show continues to veer away from parody as McMahan insisted it would.
So this entity can create items from air molecules. That's cool, but doesn't replicator do that, in effect at least? Why does Mariner use this wish-granting entity to do something the replicator can give her? The easy answer is that there's limits built in which means a tricorder is not available to be replicated. That's a worrying thought, because it means Starfleet might be undersupplying its own personnel, not providing them with the latest advances or forcing them to utilize subpar technology.
I don't think the purple stripe is part of the joke. It just indicates that is the newest tricorder to an outside observer. The entity and Mariner know the exact specifications, somehow, to this purple tricorder that advance it further than others.
Logistically, beyond the wish for a tricorder, how is this matter-replicating entity so easily captured? It can travel through bulkheads and into a person's sternum, but not through their arms while being wrestled? Mariner was familiar with it as a "trans-dimensional energy being", so maybe she's dealt with this specific species before. She was pretty adamant that it was a bad guy who zaps people (but it never zapped her), and that it could be placed safely inside a storage pod. But then she lets it go to create the tricorder and power crystal, which uses up much of its mass.
As a smaller creature, the entity attempts to kill the Captain, whereas it only wanted Mariner and Tendi to supplicate themselves. Obviously it knows alot (tricorder designs, storage pods), so it probably recognized Captain Freeman, and by killing the Captain, it wanted to take over the ship with little recourse from the crew. I don't know if it died (I would doubt it) or just entered the Captain's body and left of its own accord at some future period (or even possesses her as some plot point in the episode). We'll have to wait and see what the episode gives us.
So, I agree with the OP. The scene could've been better handled, without resorting to a self-referential comedy skit. The idea, already present from the first episode, that these Ensigns just shrug and eye roll when dealt with the cosmic horror of the universe, is great and the bread and butter of McMahan's vision for the show. But they might want to be a little more nuanced in how that is presented at times.