But, if we’re going to get into DM advice, the way I’d recommend stopping “anything could be a mimic” from getting old is to have it constrained to a themed side-adventure, or a one-shot. For example: A wizard tower where the guy’s one weird hobby was breeding and training mimics. In such an adventure, you want to start fairly tame, but towards the end, the more outlandish and ridiculous the better.
As for the constraints on mimic forms, most of my DnD based jokes use 5e as their basis, as that’s what the majority of my audience are likely using.
A long time ago I did spend a few months experimenting with ratios to make my own cookie recipe, it makes really good chocolate chip cookies. Not very christmassy though.