Is Tyranitar weak?

I think part of the reasoning for the Dragonite vs Tyranitar difference is that between a ton of good water types,even water types that can get STAB fighting moves and resist all of Tyranitars' attacks, most folks simply have many more efficient Tyranitar counters than most ever had Laprases or other Ice fighters. Plus none of those Ice guys are just not of the same attacking strength of many of the water and fighting mons.

He is still good due to high CP or if one is in an area where Vapes and other water pokemon are more rare. And since you control his type matchups on offense he can prove very useful with his high base stats for attacking.

Beast yes. Great defender? Probably not.

For dragonites, it was tough getting a lapras to take it down. So you would go for cloysters or other attacking mons to make up for it.

For tyranitar, I assume just about everyone has a machamp already. In fact, after gen 2 came out, I made it a point to farm machop candy and evolve a new machamp with optimal movesets to take down the tyranitars that will one day plague the gyms. =)

Like Dragonite, he is primarily an attacker. He is arguably a better sweeper than Dragonite, at least against the Blissey / Snorlax matchups -- as he resists Normal and Psychic type attacks. In fact, Tyranitar resists all the quick moves from Snorlax / Blissey, and resists Body Slam, Hyper Beam, and Psychic as far as charge attacks go.

Aside from being double weak to fighting, it is weak to water and water resists a lot of its moves. Everyone probably has at least one good Vaporeon powered up at this point.

Both Tyranitar and Dragonite are weak defenders in the sense that both can be slaugthered by their counters, the difference is that Figthing types are more common than Ice types, and the fact that back when we needed counters to Dragonite (we still need them btw), the game was much younger and we had less time to get those rare ice types, and now we have had longer to get those more common figthing types, so it's much less of a big deal. Regardless of this, both of them are first and foremost attackers, and they are excellent at doing that job.

Think of tyranitar as a buffed Rhydon.

Machamp and Heracross are really good attackers that you'll use against a lot of defenders, such as Blissey, Snorlax, and Rhydon.

Lapras isn't such an amazing attacker and you'll really only pull it out for Dragonite.