This is apples and oranges.
What Harmon etc did was protected speech (falling under parody and easily meeting the relevant tests for such a defense).
What McDonald's are doing is more complicated; they're clearly marketing their own sauce (which existed decades prior to Rick & Morty), but they're also clearly leveraging the popularity of the show to do so. They had no obvious plans to bring it back (perhaps alongside the live-action Mulan remake, but that's not for a while anyway). There is an argument, however tenuous, for some remuneration to whoever ultimately owns Rick & Morty (who can then argue with their employees who gets what).
Even with a sale price in pennies, a fraction of 1% on all global sales of this sauce would make you a very rich cunt. So there's a motivation to both the stars and the producers to get something back here, but it's not a matter of copyright or trademark, it's a more complex matter of whether the return of the sauce and the popularity of sales thereafter can be attributed solely to this one show (and one episode of one show, not a protracted campaign). At the heart of that is how much the show actually did before social media took over and blew the issue up - much harder to quantify. Would Ishiyagi Mootsubishi expect a payment from FOX for every Simpsons episode and piece of Steamed Hams merch sold if ratings climbed? Would he like it, sure, who wouldn't - but then again isn't the appropriation of memes the same thing as reposting images? Shouldn't Littlefinger then get a slice of 4chan's ad revenue for every Big Guy thread served? It's a complex issue of rights and costs (and one which 4chan originally had to argue against, along with other internet sites where images were posted without permission).
What it's not is a clear-cut copyright issue.