That depends on the fighter's actual height. At least half of the fighters on the roster can switch between two weight classes b/c of their body type.
For ex: Conor is 5 foot 9 inches. That doesn't mean he is exactly 5 foot 9 inches. He could be closer to 5 foot '9.9 inches.
TJ is listed as 5 foot 7 inches. He could also be closer to 5 foot 7.9 inches.
According to the
fight matrix chart which takes the average height of the top 50 fighters in each weight class, T.J. is in between the Bantam weight and Feather weight class. You could say T.J. should move up to feather weight. But the same data shows Conor should actually be fighting at Light weight so a move up weight be Welter weight against Tyron Woodley.
Another words, if you shit on TJ for challenging in a lower weight class, then you'd have to shit on Conor's entire career at feather weight.
Also if you count Conor's fight against Nate Diaz as a true 170 fight, then you're a true noob. Telling two 135 lb fighters that they don't have to cut weight and can stay at their walking weight of 150, doesn't mean they can hang with other 150lb fighters...Don't be a noob.