The best NTR has a decently strong MC and the FMC falls/ get corrupted DESPITE that. It plays with the powerlessness of the MC despite their physical and mental strength. If the MC was a pathetic twink the "fall" of the FMC would not be impactful cause MC would be a bad choice for a partner anyway.
I'd argue it's not very impactful either way -- no matter if the MC "appears" strong or weak, if at the end of the day their partner chooses to abandon them in face of trouble/temptation/whatever, then it just means that in hindsight the supposed bond was little more than lip service and the MC's supposed strength(s) meaningless, if at the end of the day the MC didn't have what it'd take for their partner to stick with them instead of opting for the alternative. They're ultimately evaluated as "insufficient choice for a partner" in both scenarios.
If there's anything different, it is that this setup makes the MC even more of a pathetic failure than that proverbial "pathetic twink", because they can't even hide behind "
well i don't have anything going for me so no wonder my partner didn't choose me".
Same for the chemistry and romance. It doesn't matter how many "ILY"s are dropped, if two weeks or months later the person declaring all that is readily fucking other people. Something that falls apart at first obstacle simply wasn't substantial to begin with, no matter how much people involved were in love with the idea of being in love.