It seems like this would be OK for the ground pigtail, and probably OK for the hot and neutral too since the receptacle is ony 15 amp capacity. But I wonder if there are safety issues I'm not considering and what the electrical code has to say about it.
You must use a 12 gauge pigtail here
A 14 gauge pigtail on a 20A circuit is a (very short) tap conductor:
Tap Conductors. As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.
As a result, that 14AWG pigtail violates NEC 210.19(A)(4) Exception 1, which prohibits the use of tap conductors to feed receptacles:
(4) Other Loads. Branch-circuit conductors that supply loads other than those specified in 210.2 and other than cooking appliances as covered in 210.19(A)(3) shall have an ampacity sufficient for the loads served and shall not be smaller than 14 AWG.
Exception No.1: Tap conductors shall have an ampacity sufficient for the load served. In addition, they shall have an ampacity of not less than 15 for circuits rated less than 40 amperes and not less than 20 for circuits rated at 40 or 50 amperes and only where these tap conductors supply any of the following loads:
(a) lndividual lampholders or luminaires with taps extending not longer than 450 mm (18 in.) beyond any portion of the lampholder or luminaire.
(b) A luminaire having tap conductors as provided in 410.117.
(c) Individual outlets, other than receptacle outlets, with taps not over 450 mm (18 in.) long.
(d) Infrared lamp industrial heating appliances.
(e) Nonheating leads of deicing and snow-melting cables and mats.