To understand what makes a good Mixed Wall, I'll first define it.
>A mixed wall has excellent stats in both Sp. Def and Def. This allows it to take both Physical and Special attacks consistently over a game. This means that it can deal with a variety of different attacks. Unfortunately, there are only a few walls that can fit this category.
Basically, to sum it up, the Pokemon must have good base stats in both Sp. Def and Def. This means that they must have atleast 100 base Sp. Def / Def. Many of them also have decent base HP (it usually varies around 70 ~ 100).
Now, obviously, stats alone cannot define a mixed wall. It also matters about typing, movepool, and more. For example, Articuno does have the stats and potential, but its typing is awful. Its combination makes it 4x weak to Stealth Rock (a common entry hazard). This means that it will be taking about 50% of its health per switch-in. This is not what you want on a mixed wall. Articuno has better offensive typing, so it works better as a sweeper.
Examples of good mixed walls include Ferrothorn (great typing with little weaknesses and a lot of resistances / defensive stats to compliment it), Umbreon (pure Dark-typing with only 3 weaknesses and 3 resistances / excellent HP, Def, and Sp. Def), and Hippowdon (pure Ground-typing / good HP and Def and decent Sp. Def / reliable recovery in the form of Slack Off).
Hope I helped!
Although Articuno has a high Sp. Def stat that distinguishes it from many other Flying-types, Articuno has 4 weaknesses to some of the most common types (Rock, Fire, Electric, and Steel).
This alone is a big problem because it means that it takes 4x to Stealth Rock.
Here I'll quote from my friend, who introduced me to competitive battling (he explained this same problem to me):
While it is very bulky and quite strong, it is incredibly difficult to fit on a team due to its 4x weakness to Stealth Rock and mediocre synergy with many entry hazard removers.
Articuno's best sets combine both its offensive typing and its great defenses, which allows it to run a SubRoost set with Hurricane and Ice Beam / Freeze-Dry or even an Agility set.
This means that you could run a Mixed Wall set, but you're not maximizing its full potential.