I know you've mentioned this issue several times before, but this is the first time it has annoyed me. In the past, this was the kind of nutty thing Castle would go on about just to get a reaction out of Beckett -- and it was amusing to watch. But the fact he was serious about the Bigfoot stuff was ludicrous. They even had Alexis acknowledge that he believes in all these weird & crazy things, so it's not just our perception of the character that has changed -- it is his character that's changed, and not for the better IMO.
I'm surprised you'd react that way to this and not to the knockoff of The Ring
they did just a few weeks ago, where Castle was afraid he'd die because he watched a creepy video. At least believing in cryptids isn't totally unrealistic; there have been cases where previously undocumented animal species have been discovered, like the coelacanth. But in that one, not only did he believe in ghosts, but he seriously believed that a danger blatantly copied from the plot of a movie could be genuinely real.