I suppose each user can leave max one sticker per profile and update it whenever

the price of a usual post seems reasonable for MVP, treat it like a comment on the bio maybe

Could get fancy with pricing model to the stickers to reputation somehow. Or maybe to thwart abuse if becomes needed.

I was thinking purely cosmetic, maybe even free?

nullcount

Reputation "Stickers" for SN?

This is kinda like an alternative method of calculating the trust of a user, which SN already does for weighting zaps in determining content ranking.

Interesting idea! What do you propose it costs to vote on an emoji for a user? I assume you can change your vote over time?