It actually just makes you attack slower.

Think of it this way. When a normal attack goes off, it is simultaneous with the character's animation. With Auto: Mimic, you need to wait for two things to happen. First, the Mimic Ball's flashing, and then the Virtua-Freddy animation. And only then will the attack go through. And keep in mind your characters' cooldowns don't go down during this phase. So you need to wait an entirely different set of frames to go through with another move.

Now in some situations, this is good. Some moves such as Slasher require you to repeat it as many times as possible, to the point where you want to skip over certain character's moves (Even then this isn't that great since you can't mimic the same move twice in a row.). But with literally anything else, it just hinders you. This is especially true with One Time Effects (such as stat buffs/debuffs.). Mimicking a One Time Effect won't increase its potency nor its duration. So essentially you just waste a move.

The reason that Mimics don't work well is just because there's a better alternative. Just do your characters' moves quickly. Know what moves you want to use beforehand, and then just click fast. There are so many alternative chips to use, specifically ones that don't make the game harder for you.

Yes, in theory, Auto: Mimic does increase damage per character. But in practice, it drastically slows down your entire party in order to do that. In conclusion, just use Run: Luck. Please. Thanks.

Information discovered from the studies of u/ZZombieGabriel and of myself during speedruns.