... I don't think this would really be classified as NTR.
From my understanding NTR is usually from the perspective of the person getting cheated on. Like the whole fetish revolve around the despair the person feels after having their significant other/person they have a crush on stolen from them in front of their eyes. If anything this would fall under a more general fetish of cheating which even then it wouldn't since, you know, Ash isn't fucking his mom. If they were having sex with an adult Misty or like Dawn or whatever maybe but Ash's single mom getting laid doesn't really fall under that.

Base +6, Romi the Byzantine +7, GodManZ +8, kageganon +6, 绯flamehaze +6, C.Edouard +7, Jeremy69 +6, dragolem +15, Originality +6, ezequiell +29, joseliski -6, SkullDraecos +6, Deep33 +7, and 4 more...

Ok im just going losing my boner laughing when i see mewoths face.

Base +6, meficrow +12, ghostvortex100 +6, FullestDay +26, Und3adgam3r +7, 绯flamehaze +6, Johann74 +5, Clamstatic +6, Jeremy69 +6, dragolem +15, pantonious +6

With all that sex I'm surprised that James didn't impregnate Jesse and Ash's mother and since the father is nowhere to be seen the mother should divorce the father and Mary Jesse and James

Base +6, Und3adgam3r +7, dragolem +15, pantonious -6, PregnantWhoreFucker +6, Deep33 +7

Wow, didn't know NTR stands for Nintendo's Team Rocket

Base +6, anonymousMember +6, squashmonkey +6, Romi the Byzantine +7, kageganon +6, Und3adgam3r +7, Johann74 +5, twothousand +6, grianj +6, C.Edouard +7, Jeremy69 +6, dragolem +15, hereforpandaonly +6, and 10 more...

@ Confussed-Oddish
I think that, now that the set's complete, we can definitively say that the one getting NTR'd is Meowth.

Base +6, squashmonkey +6, Romi the Byzantine +7, Und3adgam3r +7, C.Edouard +7, Jeremy69 +6, dragolem +15, hereforpandaonly +6, elistraee +8, IaY +10

This pairing (trio-ing?) is definitely a breath of fresh air after the 9436635th Prof.Oak/Delia work.

Base +20, Und3adgam3r +7, delta82112 +6, Clamstatic +6, Jeremy69 +6, dragolem +15, d20master +7, Doutey +11, Karkhouv +6

@gigasgmx
That would imply Meowth want's to... what the hell anon.

Base +6, C.Edouard +7, dragolem +15

You say NTR, I say the most refreshing and interesting throuple dynamic in Pokemon and one that I now want to see even more of in the future.

Base +6, C.Edouard +7, dragolem +15, hereforpandaonly +6, SkullDraecos +6, pantonious -6, satellitea +9, Deep33 +7, Karkhouv +6, JackRyan86273751 +4

It's my personal opinion, but now Team Rocket is like [another family] to ash, so this is more of an incest than an ntr.

Base +6, elistraee +8, Deep33 +7

Prepare for trouble, cuz we've been banging your mom
And make it double, cuz that pussy's the bomb

Base +9

Since it isn't showed that she left Ash for the New Team Rocket, this isn't NTR, just cheating.

@Confussed-Oddish Mother/Sister/Relative NTR have existed since the ancient-age. Ash fucking his mom or not matters little.

Base +13