I judge this link as INSUFFICIENT INFORMATION to rate Trump's decision as a positive or negative thing. The key words (taken from the link - Snopes) are >>
"would have plausibly prevented certain classes of mentally ill people from purchasing firearms." There is no information provided as to the type of classes these involve.
For instance, for all we know, it could be classes of people who are very forgetful, have agoraphobia (afraid to drive over large bridges), excessive love of animals, or have fetishes for western clothing. All types of mental illnesses, but which are harmless, and have nothing to do with crime or guns.
In the past, some politicians have take actions to set up opposing politicians to make them look bad. Is this one of those.? More information would be needed to assess the whole thing.