I think that's a good data point, but it's just one. Mountcastle has been in the majors for 1638 PAs, he's 27, and prior to 2023 he had a .774 OPS, in 2023 he had a .779 OPS. Fangraphs lists a bunch of projections, all of them have Mountcastle between .758 and .789. They're just projections, but nobody is betting on a big breakout. Doesn't mean it can't happen, but I don't know if it's likely.
How well do unexpected 2nd half splits actually stick? In 2022 Nathaniel Lowe had a .964 OPS 2nd half, and a .771 coming into the season. In 2023 he had a .775. Eloy Jimenez had a .948, an .822 coming in, and a .758 in '23. Trayce Thompson had a .947, coming off a .688 career, and then a .579 '23. Joey Menses had a .930 as a 2nd half '22 rookie, then a .722 in '23. Coming into '22 Bo Bichette had an .850, 2nd half of '22 he had a .921, then an .814 in '23. Jake Fraley had a .656 going into '22, a .903 2nd half of '22, and a .783 in '23. Vinnie Pasquantino had an .899 as a late '22 rookie, then a .762 in '23.
I'm sure you could find counter-examples, but for the most part the plexiglass principle applies. Or you could call it the SI jinx. Players who play unexpectedly well (and often are recognized for that with things like magazine covers) tend to regress to career marks. "Tend to" doesn't mean that's destiny, but I don't know that I'd count on Mountcastle being a top-10 offensive player in '24.