I don't really see the point in making an entire tag system in-game to point out that players might be alternate accounts. The only situation in which being an alt is illegal is when farming low belt areas. Calling out an account for playing a certain way or being skilled beyond what a belt's skill level is implied to be is a waste of time and is frankly just inappropriate action. No one wants a permanent tag on their account that is, for the most part, subjective.
Don't even get me started on your idea of being allowed to report "dirty" playstyles.
Just give an overview of the general rules and reportable offenses at the end of the tutorial and let new players understand how to report actual problems. This idea is a waste of effort and a slap in the face to anyone victim to this tagging you suggest.