A reasonable opposition party would declare the pardons invalid. Is that a valid interpretation of pardon power, does that undermine the legitimacy of our laws? Maybe, but not nearly as much as not punishing obvious and proud criminals does. That's the point of the rule of law, remember? It creates legitimacy, and therefore stability.
The current USA opposition party doesn't really do anything when they actually obtain power. They bark a lot when they're out of power, but as soon as they are back in power, they just go limp, forgive and forget, for the sake of unity or something.
> A reasonable opposition party would declare the pardons invalid. Is that a valid interpretation of pardon power, does that undermine the legitimacy of our laws?
No, in the USA the pardon power belongs to the President. Only a constitutional amendment could invalidate pardons.