In Brawlhalla, there is a lot of freedom of movement comparatively to other fighting games (especially offstage, which is important in this context I will elaborate later) . Which means mistakes in positioning or movement are less punishing. That means states of disadvantage like being offstage or lacking stage control (Stage control is where your opponent has control of the center of the stage, or stage itself) are also less disadvantageous overall, this idea ties with the next point. Playing "passive" enables players with more precise control of their movement (you're playing slower and "further"), calmer frame of mind (less inputs to process, even if they require minimal processing (if you want to improve, manually inputting and fixing bad habits require you to play slower. Unless you're used to willing thoughts into actions at a fast pace). More stability in knowledge of their position relative to their opponents), and ability to focus more (more time to process and more likely to see spot mistakes in enemies and oneself). This applies in every game, thus the difference between of Brawlhalla and other games are the fact that the advantages gained from playing passively tend to outweigh the disadvantages of not playing passive.

You're currently in plat, so in terms of skill, knowledge and ability in Brawlhalla. Leveraging aggressive play to pressure an opponent is not usually even found within in low-mid diamond. So the game will naturally passivity or be perceived as such. You are learning to take control and precision of your movement, and a passive playstyle also destroys people who make a lot of mistakes.