to simply answer your question, i think acquirers do try to make sure merchants have the right mcc on a "best effort" basis. but there are just too many ways that it can go wrong, and the best effort isnt actually much because it isnt profit making. even in a country like japan, i have seen restaurants coded as automotive or computer services or real estate

when i set up an online merchant account for my mum to sell her handicraft, the mcc is whatever i declared and nobody checked. i dont recall the specifics, but i remember that the mcc is wrong as the selling platform only allowed a subset of mcc available and i couldnt find one that i thought was right. and when my brother started selling some other things via the account later on, he was advised that we dont need to change the mcc as my mum was still selling her handicraft, although my brother was selling way more than my mother by then. idk if this is what usually happens with a traditional acquirer or that these 3rd party platform/payment service providers have just made the situation worse

there is also no standard definition for each mcc category, so it is subjected to interpretations and sometimes visa and mc give different mccs

in terms of whether merchants have incentive to falsely declare their category, i am not 100% sure, but did hear that certain types of merchants are deemed more high risk and may pay higher interchange