Its not technically a bad tackle, its more of a very ufortunate sitaution, both of them went in hard for the ball, and both of them pretty much got part of it, its just a tragic accident that the kid got his leg stuck in dangerous angle.
But the sitaution itself, if he hadnt got stuck between Nainggolans leg in such awkward angle ( the tempo and force of it made it worse), I'm pretty sure the ref would prolly given freekick to Nainggolan, since Mattiello was one standing and coming in more dangerous angle and slightly later on the challenge.