There's sometimes different levels of mothballs, to be fair. In the case of the Tennessee above, she and her sisters of the Big Five semi-class were kept in reserve through much of the 1950s, but realistically there was never any intention to ever reactivate them again- the battleship's day was done, and certainly if that was the case there was no interest in reactivating battle-scarred, three-decade-old dinosaurs in particular. Had those ships been slated to go back on active duty, it would have meant a calamity had befallen the U.S. far beyond anything imaginable at the time.
The result? Ships that are maintained, but not exactly kept in perfect shape. After all, these aren't even manned by skeleton crews- they're inspected on a rotation basis, with no one really aboard full-time, so rust and undergrowth will happen far faster- partly due to lack of a crew to maintain, partly because they aren't moving about anymore.
Note however that in comparison, the Iowas at this same point were still considered possible to reactivate (though with no concrete plans to do so- even the New Jersey's Vietnam reactivation was a sudden and ad-hoc process), and were maintained far better as a result- after all, the better condition the ship is in, the faster it can be reactivated, so some ships like these got better treatment as a result. (One can expect that was a major factor in the reactivation of the class in the 1980s- after all, if they were in wretched shape like the Tennessee up there was, why bother at all?)
One other minor factor to consider, looking at the 'modern' reserve ships earlier in the thread- color camera shots show rust and wear far better than a black and white image does, so those famous shots of reserve fleets postwar in San Diego, Suisun Bay, Philadelphia, etc. look a lot more trim and clean than those ships above- because we can't see brown rust and such as easily. I know that sounds stupid, but it really does end up being the case- looking at images of the ruins of Japanese ships postwar at Kure, a B&W shot of the carrier Amagi on its side shows a ship laying on its side bombed out- but a color shot shows the extent that a year of that condition has done in terms of rust and decay far better.
