I don't know whether the team made the Ultra Wormhole moveskill available only to five legendary Pokemon for balancing purposes or because the only Pokemon they could think of were legendary, but either way I recommend giving it to Kartana anyway.
I don't think it is overpowered, partially because Ultra Wormhole has a large cooldown and the wormholes spawn naturally quite often in 1.12. However, my main reason for not finding this broken is that Kartana is a late-game Pokemon. By the time you have the means to get a Kartana, you likely own a flying mount that can get you to the naturally-spawning wormholes. You may even have the means to get one of the other Ultra Wormhole users by that point. Kartana only spawns at level 70 in Ultra Space, so your Pokemon need to be high enough level for level 70 Pokemon to spawn in the wild. Other than that, Kartana rarely appears in hard-to-beat 5-star raids. Plus, unlike the five Pokemon that get Ultra Wormhole at the moment, Kartana isn't a flying mount. You'd need a separate Pokemon to get to the portal, so it would be an inferior Ultra Wormhole user to the five legendaries.
I would like it if Kartana gains access to Ultra Wormhole.