This is every Pokémon that can learn Copycat and/or Mirror Move (as they're essentially the same move) in ascending order, from lowest to highest speed.
/nds (copycat|mirror move), speed asc (111 results)
Bonsly, Cleffa, Igglybuff, Azurill, Jigglypuff, Togepi, Happiny, Sudowoodo, Clefairy, Marill, Togetic, Flabébé, Oshawott, Torchic, Wigglytuff, Wooloo, Swirlix, Azumarill, Chansey, Hoothoot, Skitty, Swablu, Floette, Blissey, Combusken, Ducklett, Eevee, Pidgey, Clefable, Dewott, Farfetch’d, Fennekin, Mime Jr., Riolu, Spinda, Starly, Sylveon, Toucannon, Vullaby, Fidough, Flareon, Glaceon, Pikipek, Smoochum, Sprigatito, Umbreon, Vaporeon, Zorua, Purrloin, Mr. Rime, Noctowl, Samurott, Spearow, Honchkrow, Pidgeotto, Slurpuff, Braixen, Doduo, Florges, Shroodle, Smeargle, Tandemaus, Trumbeak, Altaria, Blaziken, Mandibuzz, Mesprit, Staravia, Togekiss, Floragato, Buneary, Samurott-Hisui, Taillow, Dubwool, Delcatty, Flamigo, Lucario, Mr. Mime, Chatot, Murkrow, Squawkabilly, Oricorio, Dachsbun, Jynx, Leafeon, Minun, Plusle, Mimikyu, Sigilyph, Swanna, Blaziken-Mega, Fearow, Mr. Mime-Galar, Staraptor, Pidgeot, Delphox, Lopunny, Zoroark, Liepard, Dodrio, Espeon, Grafaiai, Maushold, Lucario-Mega, Pidgeot-Mega, Meowscarada, Marshadow, Swellow, Jolteon, Tapu Koko, Lopunny-Mega
Most of the Pokémon with the lowest Speed here are baby Pokémon that couldn't feasibly be used in an actual battle, so better (but not very good) options would include Sudowoodo, Clefairy, and Wigglytuff. This strategy wouldn't counter Trick Room, just using the last move done by the opponent, which is generally less effective than using one of your own moves. Simply using Taunt, Circle Throw, etc would be more useful and have more practical uses.
