My ex-wife says giving my son too many fruit snacks (too much sugar) caused him to have a meltdown at school the next day. Is this even the slightest bit likely? He is 5 and in pre-school.
Sugar does not cause hyperactivity.
However, if a kid learns from his parents that he is expected to be hyper after eating sugar, well, you can guess what will happen when he gets sugar. Even with that trained response, I don't think there's any plausible reason that sugar would cause bad behavior the next day.
There are lots and lots of reasons to restrict the processed sugar that kids consume, but behavior problems the next day isn't one of them!
There was a comment that your wife is looking for something blame. Maybe she is looking for something that she can tell you to get you to stop giving him sugar.
Perhaps you should stop giving him sugar.