ex mercury sag sextile mars libra. is that good or bad? or merc conjunct uranus
Generally this is a GOOD thing. It will make the inherent strength and sterness of the malefic be more generous and good-natured.
The opposite, having malefic planets influence benefics, is generally BAD. It makes the inherent goodness and generousity of the benefic has self-serving aims, or to become restricted and less abundant.
Mutual aspects are, of course, two way streets. Interpret accordingly, the affect to the malefic is beneficial, while the effect to the benefic is undesirable. Similarly with conjunctions, which are always mutual effects, as in your second example (Mercury conjunct Uranus)