It doesn't have to be viable in competitive play to be a good champion,
Being good at pub-stomping doesn't make a champion good.
Especially against a team with at least 3 AD's, that would remove a total of 270 AD which could mean life or death for a team mate, and a won team fight.
Yasou's a melee carry.
He has Wind Wall.
Way more than 270 AD potential of damage-blocking for his team.
It just seems a little too supportive for Tryndamere in my opinion. Not saying he's OP but it doesn't fit his playstyle and just seems like something they slapped in there for a W that turned out to be more than they'd hoped.
In a team fight, Burst > Sustained, Defense > Squish, Range > Melee, CC > Damage, AoE > Single-target.
Tryndamere's not a solid champion that's teetering over the edge with Mocking Shout.
Mocking Shout barely allows him to bring anything to the table, even layered on top of his other strengths.
And you could say that the AD debuff isn't much but, I doubt if you've played at least 6 months you've never had a game with Tryndamere on your team that W'd and you got out with a sliver of health.
I've been playing LoL since Poppy and Gragas were released and I don't remember that happening.
Tryndamere in general is one of the most underwhelming allies I've had. To be fair - mocking shout is the definition of invisible power, it's pretty hard to actually feel a Tryndamere is helping with utility/supportive stuff.
But he's also not exactly Xin'Zhao levels of totally ruined the enemie's AoE team-comp, Yasou levels of blocked all 5 ultimates, Fiora levels of Riposted a Nami ult and then Janna-healed the entire team, Wukong levels of pro-baited a Lissandra ult with clone then gave an epic initiation, Fizz levels of a perfect 5-man ult giving amazing initiation potential, or Talon levels of just deleted someone in the blink of an eye.