Hidden power

Sadly, even if togekiss got dragon hidden power, it wouldn't be a dragon slayer. as the dragons already greatly out DPS togekiss, even in cloudy weather rayquaza beats its DPS by ~6. and also resist palkia's other 2 charge moves.
Its a lot like gardevoir, even if it got a fast move effective agaisnt dragon the dragons just outpace it so much not much other than another dragon can come close (the only 2 exceptionswe have now/near future are dialga as it isnt weak to dragon or fairy thanks to steel type, and Rayquaza who's double weakness to ice and the existence of mamoswine)

Could be fun to play around with if you've got a large group and just want to play around for variety, which in truth is the core of pokemon

The only other type that HP can't be, aside from Fairy, is Normal-type.

On the topic of Togekiss, it's true that it won't be the highest DPS option against any Dragon, really. Ice-types and other Dragon-types will dish out damage much faster. However, since Togekiss has immunity-tier resistance to Dragon-type attacks, it will have significantly higher TDO, especially compared to some of our top-tier Dragons.

Regardless of not being the top-tier DPS option, I'm still gonna be using my flying puff ball to slay dragons. Why? Because fun! 😁