Answer: I WON
Explanation: We have our four main themers (whose clues were irrelevant, so I just went with general math stuff - sorry, haha): NINETYONE, EIGHTYONE, SIXTYONE, EIGHTYNINE. Now that's all fine and good, but in a puzzle centered around four two-digit numbers, to have the same digits repeated often cannot be a coincidence. In numeric form: 91, 81, 61, 89. What do these numbers have in common? In line with the title, we can all turn them around - literally 180 degrees around (upside-down!), and a new number forms. In order: 16, 18, 19, 68. Note these are now in numerical order also, hence why I wanted to keep those themers as they were and broke symmetry as a result.
Conveniently, all four of these new numbers are in the grid; the letters at those locations are U, O, M, and I. This certainly doesn't look like much at first glance, but let's abide by the title again and turn these letters upside-down (but as a package deal, so U O M I itself gets turned 180 degrees and not just each individual letter). This forms I WO∩, the answer to the meta, and hopefully something said after someone turns things around! (The Texas Rangers turned things around considerably this past season!) This might even be more clear writing the grid letters in lowercase first.
I already can anticipate one possible quibble or question: why would I have the "N" in "I WON" come from a "U" as opposed to an "N" itself (which, when turned upside-down, is also an "N"?). A mix of reasons that maybe won't pass muster, haha. First, if the "N" stayed an "N," the only real exciting thing that happened on turning the letters upside down was the "W" goes to an "M." I also thought some might try mirror-image stuff with that - I almost thought it would spell "I'M ON!" but I think the "N" would be backward. And, honestly, I just thought the "U" turning into an intersection-like "n" looked cool lol.