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Mementos as event rewards
I won't try to speak for "the vast majority of players" like you see in other events threads, but I'll speak for myself:
Mementos suck as event rewards. I've been playing this game for over 5 years and I see Mementos used by players no more than a handful of times a year. And that's usually when players are bored waiting for a trail to start or a boss to spawn. Even roleplayers are more likely to type out an emote than to use a memento. Your system for activating them is clunky and deliberate and most of them are just bland.
Easy answer is "If you don't like them, don't use them." but given how infrequently I see them used in the thousands of hours I've played, I'd say people collect them to collect them. How about replacing mementos with different cosmetic rewards? You have several and I have seen ALL of these in use every day:
Costumes
Hats
Adornments
Hairstyles
Markings
Personalities
Skins
Name change tokens
Outfit change tokens
Race change tokens
Polymorphs
Furnishings
Respec scrolls