Why is Lickitung not considered a powerful Pokemon?

not really, i'm assuming that it's either 76 or 67 HP.
Zen headbutt is a slower quick move, and combined with Hyper Beam, your Lickitung's moveset is pretty slow overall.
It's not bad but it's not that good either.

Lickitung is fairly average. It gets outclassed by many other Pokemon who have much higher base stats and stronger moves.

Your Lickitung has a great moveset, but its damage output is depressed a bit by its low Attack stat. For example, a Snorlax with the same moveset would do about 40% higher damage, simply from its higher base stats alone.