Should MMA be called a martial art?

Why not. It is not a traditional are form as one might perceive it when speaking of traditional/classical martial arts but it is like many systems throughout history where it is comprised of combative skills.

I would nick pick a bit from my perspective because I cannot determine any spiritual part of the practice of MMA. I would tend to discount the "art" aspect of the title but also understand that many martial or karate like techniques in a system tend to get that system lumped under the general title of karate and/or martial art.

If I understand it correctly MMA got its name because it was a mixture of a couple or possibly several systems like Jujitsu, Karate, Judo, Wrestling, etc. It is a "mixture" of what is perceived to be the best for that genre of competitive "sports-like" martial arts - what ever that truly means.

Since the "majority" of practitioners in the west tend to call everything they do that smacks of Asian systems of fighting, etc. as "martial arts" then since the majority do accept it thus then it must be accepted but rather as a new definition  and not the traditional or classic definition of martial art. Honestly, the martial arts are muddied to where the true spirit and meaning of that phrase is mostly lost to except a few knowledgable folks.

So, why not.