Oh, definitely. I think many artists have something that's comfortable and feels good to draw, or a certain selection of OCs or themes that end up filling most of the sketchbook.
I don't think it's a bad thing or a good thing -- like, obviously drawing your characters over and over again means you're very familiar with those characters, but for me at least, those fun, comfortable scribbles can sometimes become sort of artistic junk food -- not really stretching you at all, just drawing a thing you already know how to draw.
Trying new poses, new expressions, new interaction, working things out with those same OCs is productive and useful. Just drawing them over and over again because they're fun to draw is probably not as artistically useful but definitely not a bad thing -- like, you got into this game because drawing is fun, why would you not draw the fun stuff???
I think sometimes you do gotta eat your veggies, so to speak, to grow -- drawing things that you wouldn't normally draw. If you want to learn to draw environments, for example, I don't think you ever magically think "wow, now all I want to do is draw environments in my sketchbook!!" -- instead, I think you have to sit down and fight with them a bit before you understand them. And they can eventually become something you enjoy (I used to hate perspective and now love it!) but I don't think that happens until after you've made yourself sit down and learn how they work even though it doesn't come naturally.
OCs and personal stories can be a good gateway into new stuff, for me -- if a character I care about has a horse, I'm much more likely to want to figure out how to draw horses. Drawing an environment is more interesting if it's my character's hometown and I can draw him in it.
But, all of that said, to this day my sketchbook is generally a whole lot of scribbles of my favourite OCs with a few other things sprinkled in -- if it's fun and you enjoy it, don't ever feel like you need to indulge less in the fun stuff. When it comes to art, junk food isn't bad for you!! ;u;