It really depends on how assertive, and in what situations. It’s good to be assertive enough to get what you want when someone is trying to take advantage of you, or is crossing one of your (reasonable) boundaries. (I once met a lady who threw a fit when people yawned around her because they were contagious and she didn’t want to “catch” their yawn.)
It’s not good to go through life as an asshole or a bully who pushes people around for little to no reason.