Avid Trick Room user here,Mega Abomasnow has the niche of being a powerful Trick Room sweeper. It's a small niche, but a niche nonetheless.
Mega Abomasnow really doesn't impress me at all. First of all full TR teams rarely have hazard support because none of the Pokemon on the team are weak to SR. Mega-Abomasnow's SR weakness is thus aggravated on these teams. Next, in Trick Room, you either have those bulky slow Pokemon that switch in to attacks after you set up (Conkeldurr), or you have those frailer ones where you usually need a free switch but it's OK because they hit so damn hard (Crawdaunt). Sometimes you even have a combination of both (Mawile)! Mega-Abomasnow can't really switch in to anything, and unboosted 135 attack, even if it's mixed, is frankly pathetic for a TR sweeper. You're not breaking past any competent mixed walls and a lot of steel types with its power/coverage. And it takes up your mega slot. Does this thing have any redeeming factors? Well, it's pretty bulky but its typing neutralizes this... at least it has priority...
Mega Abomasnow => Unranked
That's right, I'm trying to kick a Mega off the tier list. You know why? Because this thing is only on the viability ranking because of its TR niche, yet I'd rather use unranked shit like Rampardos or even Pangoro as a TR sweeper over this.