I don't think it's useful, and usefulness is the alpha and omega of whether a tag should exist. Yes, we have other tags that are more specific than monsters, but that doesn't logically mean that all specific monsters deserve tags. We need to be more choosy than that to serve site usefulness of tags.
Yes, we have dragons, but we don't have blue-dragons and wouldn't consider that useful.
Yes, we have wish, but the problems of wishes in general, not just wish-spell-within-a-specific-game, are general interest enough to deserve a tag. It describes a common type of problem: handling the extreme power of wishes within roleplaying games.
Yes, we have undead, but that's a big category of things that often have questions about them. We might even consider at some point having
zombies or * vampires† considering how central they each are to their respective RPG subgenres and how common questions about the latter come up here, but I don't ever see us thinking that having musk-zombie is useful.
Yes, we have wizard, but we don't have necromancer. The right tag there is necromancy — the activity itself is where we see questions come up. The actual D&D subclass is not usually a focus enough by itself to transcend wizard and doesn't need its own tag.
We don't need mimic, and won't ever need it. It should be synonymed to monsters.
* Correction: We already do have zombies. Huh, how about that. It seems underused… Then again, the zombie RPG subgenre has always been a weird combination of popular yet niche.
† The only reason I haven't taken the initiative to create vampires already — and I've been tempted many times — is imagining it suddenly showing up as a thoughtless tag tax on every single question about the various Vampire: the Adjective RPGs. Then I back away from the tag edit box cautiously.