Yup. That was crystal clear in the 2016 GOP primary contest. First "Low Energy Jeb." Then "Lyin Ted" and "Little Marco." He didn't bother with insulting nicknames for those who he didn't feel threatened by.
I mean yeah, he's also just an emotionally stunted fifth grader who gets off on insulting people and sometimes does so just for kicks -- but for the most part, he deploys his name-calling and insults strategically, focusing on those he feels most threatened by.
Right now, that's Bernie and Biden, by virtue being the current leaders in the polls (Trump pretends to not believe in polls, but time and time again his comments and tweets show he pays close attention to what the polls are saying). Thus the tweeting about "Crazy Bernie" and "Sleepy Joe."
And of course Bernie really got under his skin by doing the Fox News Town Hall. From Trump's (insecure, emotionally stunted) point of view, Bernie came into his house, repeatedly and harshly criticized Trump, and got big applause for it from the live audience, in front of an at-home audience of millions -- so naturally he's furious that his people stood there and let it happen.