LogFAQs > #890854686

A fighting game is a video game genre in which the player controls an on-screen character and engages in close combat with an opponent, which can be either an AI or controlled by another player. The fight matches typically consist of several rounds and take place in an arena, while each character has widely differing abilities but each is relatively viable to choose. Players must master techniques such as blocking, counter-attacking, and chaining attacks together into "combos".
https://en.wikipedia.org/wiki/Fighting_game
Smash Brothers is listed here
Which makes sense since it fits all criteria in the definition of fighting game

Party games are games that are played at social gatherings to facilitate interaction and provide entertainment and recreation. Categories include (explicit) icebreaker, parlour (indoor), picnic (outdoor), and large group games.
Party games aim to include players of various skill levels and player-elimination is rare. Party games are intended to be played socially, and are designed to be easy for new players to learn.
https://en.wikipedia.org/wiki/Party_game
Smash Brothers is not listed here
Smash Brothers can be used as a party game with the right settings. It's easy to learn, but it does involve skill and player elimination. With the right settings (no stock limit to avoid elimination, random items to increase luck) it can be a party game. But not in the same league as games designed to be a party game.

People saying smash bros is not a fighting game are trying to be condescending and/or edgy, nothing more.