No, the difference is that there is a humanity in them that Ser Gregor lacks.
Roose? Has goals, ambitions, interests and coolshit beyond being a sick ****.
Spoiler:
Ramsay? The child of rape, forced to squat in squalor for his childhood. He has the human quality of fear, despite what Roose believes. Ramsay's fear of Roose is a humanizing quality, as is his queer, twisted affection for his Reek. He also has some form of pride in his name, the reminder that he is, in fact, a bastard drives him to rage, and he does want to inherit his father's name.
What of the Mountain? Ser Gregor wants for nothing, he has no grand ambition, he just sits in his keep save for a tournament or wartime. And how does he pass the time there? Systematically serial killing the populace, solely because he can. He disfigured his younger brother in an attempt to kill him over a toy he never cared for. He killed his sister... I dunno, just cause? Same for his father. There was no motive there, his dad treated him very well, and Gregor was going to gain daddy's mantle when he became a man-grown. He's killed something like three wives. He takes sick pleasure in the torture and rape of the innocent.
However, those actions are not really enough to raise the Mountain above the Boltons in depravity. What is, however, is the inhumanity inherent in how he goes about doing it. The Boltons? They have fears, wants, prides, and even things they care about. Roose even doesn't believe in kinslaying IIRC (And we know how Ser Gregor feels about that).
The Mountain? He fears nothing. He will charge a pike-line of men by himself, and he will break it. He will attempt to murder Ser Loras after unhorsing him, in full view of the king himself, and then turn on his brother for defending him.
What of wants, of interests beyond the atrocities committed? Gregor has none. Think back. When he isn't pillaging, raping, or murdering, he is so quiet, so unmoving, as to only be part of the scenery. Indeed, evil comes naturally to him, more than any other character in the series. Think back to Arya's observations in Clash of Kings. The Mountain swats people dead like flies, but half the time, he barely seems to notice the people he kills, or that he does it. Then think of Storm of Swords, before his fight with Oberyn Martell. "You killed Elia of Dorne." "Who?" That's right. Gregor Clegane crushing of the infant prince's skull between his fingers, then raping and murdering its mother, with the child's brain matter and blood still lathering his hands, the act that made Ser Gregor Clegane the most feared knight in Westeros, was so mundane and meaningless to him that he can't actually recall doing it. It was Tuesday for him.
Ser Gregor Clegane is unbound by any inhibitions, moral, practical, or emotional. He does what he wants, when he wants, and you never know what he is going to do. Only that it will be as abhorrent as possible.