Does Saheeli Rai's (-2) targeting Riddleform make a creature?
You'll end up with an Artifact Enchantment.

When an object copies another, even if it never was anything else, only the copiable values are copied.


  706.2. When copying an object, the copy acquires the copiable values of the original object’s characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The “copiable values” are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by its face-down status, and by “as . . . enters the battlefield” and “as . . . is turned face up” abilities that set power and toughness (and may also set additional characteristics). Other effects (including type-changing and text-changing effects), status, and counters are not copied.


[Emphasis added]

The token will also have the abilities of Riddleform, so yes, it can become a creature.