Neither Minny nor Indy were bad performances. Those two games combined were 47-79 for nearly 500 yards, 4 tds, and only 1 pick. Those aren't amazing numbers, but they're fine for a young qb without many weapons being forced to throw a ton against a good D. Mariota's biggest asset has always been his decision making, which is why against those good defences he threw a lot of completions, but had to keep it short often to avoid critical errors. He just screwed up late in Minny when he started pressing. Still, even with those two critical turnovers, his qbr for the two games was nearly 60, up at league average.
The other two games were admittedly bad. But that's not a huge sample. Like I said, he's looked far more comfortable these last 2-3 games, and I think the second half of the season will be the true test.