I do get tired of playing Pokémon every once in a while, yeah. Not just this; same goes for pretty much every other series, really. Usually, they tend to keep same, exact mechanics, and plot-wise, they're relatively similar, more or less, so you already how things are before you start playing anything new. As someone else had said, it's definitely normal.
As a kid, though, this isn't how I've felt; regardless of how much I'd play a series, things wouldn't get too boring for me, as I was actually quite easy to fascinate back then. Now, however, not so much.