Killjoy isn't dominating playrates right now because she has a busted kit. Its mainly due to her ultimate. A site-wide ultimate is something no other sentinel in the roster right now. Abilities like this inherently belong to the initiator class. Having her in your team simply lets you have access to defensive utility and a site-wide ultimate that helps executing a site. Hence why she is gobbling up the playrate of other sentinels. Here are some changes I think would benefit her in the balancing side of things while maintaining her identity.

LOCKDOWN:
Radius decreased 32.5m>>>>>20m
Windup time decreased 13s>>>>>5s
New effect added: As soon as the Ultimate is deployed, all allies within range are cured of negative status conditions.
Allies remain immune to negative status conditions while within range.
After the windup this effect ends and enemies within range is detained like before.

The goal of this ultimate change is to make it a more reactive and supportive ultimate instead of a proactive site execution tool. It can be used in response to an incoming breach or fade ultimate or counter a viper ultimate. The point is to protect allies from these sort of ultimates which facilitate counterplay, instead of actively initiating a site execute.
The ultimate's animation could be made faster so it can be deployed faster as a response.

Considerations:
Negative status conditions are those effects which appear as a red textbox on the bottom right of the players' screen.
Eg: stunned, decay, vulnerable, slowed, etc.
Flashes or blinds therefore are not considered to be negative status effects.

So that's kind of it. Instead of a initiator ultimate its now an anti initiator ultimate.
Feel free to share your opinions on this. I'm sure I've overlooked a few things.