Typing is one of the most, if not the single most, important parts of being a wall. By being an Ice type, it's not just your weakness count that holds you back, but your resistance count as well. All Aavalugg resist is ice, but the only ice attacks you see are from special or mixed attackers, thanks to the distribution of SR.
The only thing it's got going for it as a well is it's stats & moves, which are good and all, but with that weakness count, it has much less potential then it would have otherwise. It's also not the most reliable way of stopping dragons at all, not with fairies running around. It's also ridiculously easy to counter with that special defense. I'd actually say this thing is over hyped, to an extant.
+1 252+ Atk Dragonite Outrage vs. 252 HP / 252+ Def Avalugg: 153-180 (38.8 - 45.6%) -- 82.4% chance to 2HKO after Stealth Rock and Leftovers recovery
Alright, it can take on the average dragon dancer, but still not not as well as fairies can, and it can't do much to stop MegaZardX or DD MixMence.
Assuming SR is off the field Mega Charizard X fails to OHKO with +1 Flare Blitz and is OHKO'd in Return by Avalanche, factoring the recoil damage.
As for DDMixMence, technically that's a mixed attacker so Avalugg shouldn't be your immediate answer for that.