He's likely a full fledged member of the Ultra Guard now, but is still relatively weak.
He's only at his most powerful in his Nakama Mode, and even then we've seen clear evidence that that only makes him, maybe, equal to the other older Ultras.
Even then we've seen clear evidence that he only reaches that point when he's using his most devastating and dangerous powers, otherwise he is no where near their level.
Mebius' real skill seems to come from drawing power from friendship, and from being able to merge with other Ultras, so this definitely makes sense. He's not really a combat Ultra, not on his own. He's more a support type.
We see the true, clear difference between him and the older ones both in his duel with Leo, where he was bested at EVERY step, and later against Glozam.
Mebius went up against him and the worst he could manage was blowing a hole in him-otherwise he was completely dominated.
UltraSeven, when he showed up, took on Golzam and HE was the one to do the dominating, to the point where it's conceivable he could have killed Glozam himself...but that would have ruined the message of the episode.
I'll note, this is especially notable because UltraSeven is critically weak against extreme cold-and Glozam *IS* extreme cold.
It's one thing I respect Mebius for-it did not attempt to try to make the newest warrior look like the strongest thing evaaaaaaaah.
Mind they have more then made up for lost time with Zero, but that's neither here nor there.