A "Pikachu clone" is basically described as members of the "Electric Rodent" family that came after the Pikachu line but have many similar characteristics nonetheless. Mimikyu is not a Pikachu clone by any means. The best way to determine if a Pokémon is a "Pikaclone" is by noting that it fits all of the same criteria simultaneously:
1. The Pokémon in question are Electric-type and based on rodents. Just as Pikachu is based on a mouse, Plusle and Minun are based on mice (with some rabbit features, even though rabbits aren't rodents). Pachirisu and Emolga are based on squirrels, while the ladder is more specifically based on a flying squirrel. Dedenne is based on a gerbil; Togedemaru is based on a porcupine; Morpeko is based on a hamster; and the entire Pawmot line is based on marmots, which are rodents that are very closely related to squirrels. Mimikyu is not a rodent but rather a ghostly abomination that is simply wearing a Pikachu costume. It is also not Electric-type but rather Ghost/Fairy.
2. The Pokémon in question have cheek pouches similar to Pikachu that store electricity. Plusle, Minun, Pachirisu, Emolga, Dedenne, Togedemaru, Morpeko, Pawmi, Pawmo, and Pawmot all have Electric cheek pouches on their faces. Most of the time, their Pokédex entries or descriptions in whatever media they appear in would even reference the check pouches in some form or fashion, or sometimes even the Ability Cheek Pouch. Mimikyu simply has fake cheek pouches on the costume. Whether or not Mimikyu's true form has cheek pouches is unknown, but also very unlikely.
3. The Pokémon in question must belong to the Field and/or Fairy Egg Group. According to the lore, Pikachu clones are all said to be closely related, even though they are different species. By including them in the same Egg Groups, this ensures that every Pikachu clone can breed with one another. Plusle and Minun are part of the Fairy Egg Group only. Emolga and the Pawmot line are in the Field Egg Group only. Pachirisu, Dedenne, Togedemaru, and Morpeko are all part of both the Field and Fairy Egg Groups. Mimikyu is part of the Amorphous Egg Group, which is characterized by Pokémon that lack a body shape. Thus, it isn't related to Pikachu at all and is most related to other ghosts and blobs, not rodents. Note that its Fairy typing is not the same as being part of the Fairy Egg Group.
4. The Pokémon in question must be able to learn the move Nuzzle by leveling up and must have had this as a signature move before Generation VIII. Plusle, Minun, Pachirisu, Emolga, Dedenne, Togedemaru, Morpeko, Pawmi, Pawmo, and Pawmot all naturally learn the move Nuzzle. Sure, a few other Pokémon also learn Nuzzle, such as Hattenna, the Boltund line, and the Toxel line, but they don't fit the other criteria I listed previously. Also, before they were able to learn this move, Nuzzle used to be a signature move for all Electric rodents, proving that they are meant to be related. Mimikyu can't learn the move Nuzzle at all. In fact, Mimikyu can only learn three Electric-type moves via TM, which are Thunder Wave, Thunderbolt, and Thunder.
Notes: Sometimes, the fandom would consider Pichu a "Pikachu clone." I disagree with this because Pichu was created to evolve into Pikachu, which makes it part of the same evolutionary family rather than a "clone" meant to be based on Pikachu. If anything, Pichu is best described as literally a baby Pikachu or Raichu rather than a Pikachu clone. Even more outlandish, some fans would even attempt to say the Marill line are Pikachu clones when, despite being based on rodents, they fit none of the criteria listed. Mimikyu can sometimes be featured in artwork with the Pikachu clones. I think this is done to show that Mimikyu is "related" to Pikachu and friends based on its attempts to be Pikachu, but it otherwise fits none of the criteria of an electric rodent, meaning that it is not a Pikachu clone by any means. Lightning shaped tails are also not a criteria, as this doesn't apply to Dedenne, Pachirisu, Plusle, or Minun. Base stats, while they used to be reliable criteria, considering that the first Pikachu clones consistently had 405 base stats, Emolga broke the trend with a BST of 428. Afterwards, the BST only increased and the current Pikachu clone, Pawmot, has the highest BST at 490, which is even five points higher than the Raichu that it's based on. As for evolutionary stages, the Pawmot line seems to be the only family of Pikachu clones that actually has more in common with Pikachu by having a three-stage evolutionary line. The rest don't evolve at all.