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I personally love Dynamax as a mechanic, I think of it similar to Totem Pokemon in the way it can create 'boss battles'
In competitive play, it's a trump card that can apply to any Pokemon without taking up an item slot, and doesn't make any Pokemon in particular better than any other, unlike megas/Z-Crystals.
I think people just don't like it because it's kind of fetishy with like the 'giant' peeps, which is admittedly very cringeworthy.