Thoughts about Re-Skin

For D&D 5e, what is your opinion on treating races and classes as mere stat blocks, without necessarily being tied to their default lore — the classic “re-skin”, since it makes some sense in lore?

For example, a warlock who made a pact with an extraplanar entity, but instead of using the Warlock stat block/class mechanics, uses the Sorcerer class mechanics.

Or a Paladin who swore an oath to a cause rather than to a deity, but uses the Cleric class mechanics.

Or even a complete racial re-skin. For example, the Emerald Dragonborn has resistance to psychic damage, can deal psychic damage in a cone, can communicate telepathically, and has a limited flying speed. Mechanically, this seems like an ideal stat block for a player who wants to play a Mind Flayer-like character.