Aberrant creatures are native to or corrupted by the Far Realm. But what other than origin makes something thematically of the Far Realm?
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Lovecraftian thematics, mostly.
Does it have tentacles? Can it be described as "squamous", "rugose", "cyclopean", "fetid", "Lovecraftian", or "obscene"? Does it have ichor? Is it beyond mortal comprehension, or a manner of thing which mortal men would do better to remain ignorant of? Does it shatter one's notions of a tidy, understandable universe?
If you answered "Yes" to one or more of these questions, it may be something from the Far Realm!
Back in D&D 3e, before the Far Realm connection was really made explicit, the aberration type simply meant that a given monster had "a bizarre anatomy, strange abilities, an alien mindset, or any combination of the three". In practice, this usually just meant anything that didn't fit into the various other creature types (humanoid, magical beast, undead, etc.).
If it looks and/or act like buddies of Cthulu then I say the creature is a good candidate to be considered abberant.
On a more serious note Cthulu mythos creatures are not so much evil but indifferent to the existence of normal people. They simply don't consider us which often leads normal folks getting the short end of the stick as these creatures live their existence. Often their very presence is harmful by their alien nature. Their actions are incomprehensible and thus viewed in the best case as malign in the worst case with horror and insanity.