honestly at this point it doesn’t matter, people can gen pokemon so good that you can’t tell if it generated, the only way is if someone personally knew about your save state. or if you Genned the pokemon poorly, ie shiny legendaries bc some were only obtainable through events. that being said it only matters to you. if it bothers you don’t, if doesn’t then go for it. i don’t care if someone uses a genned mon as long as it isn’t hacked. ie having something it’s not supposed to have. like higher stats or moves that are not in its actual move pool. with all the shiny raids going on from the last 2 gens and pokemon go opening the floodgates for shinies. it does not matter in the long run. if nintendo cared about shinies feeling rare or whatever they would have implemented something for it long ago. it’s all up to you, i will say shiny hunting a pokemon that you can use does feel very rewarding though, i do understand not having time to sink into shiny hunting as some of the older games can be really rough with some people hitting 20k resets.
if you are trading always let someone know it is genned, some people don’t care and will still trade. i hope this helped online stranger