Yes you can eat cold fajitas as long as the ingredients such as the meat in the fajitas are fully cooked.
As long as you've fully cooked the fajitas and then you refrigerated them or have left them to cool down you can eat the cold fajitas without heating them back up.
I've eaten cold fajitas before and they are okay to eat cold but I prefer fajitas to be eaten when they are still hot or at least warm.
But it's not gonna harm you to eat cold fajitas although traditionally fajitas are served and eaten warm.
But if you want to grab a cold fajita and eat the cold fajita without warming or heating the cold fajita back up you certainly can do so.
Fajitas as long as fully cooked can be eaten either warm, hot or cold.