Katy is. She's actually a great singer.

Not so sure about the other one tho.

They are on the same level tbh.

And yes, they are.

Yes, they're not as good as other Pop singers vocally, but they can definitely sing.

Taylor hit every single note and slayed at her concert I saw, and Katy's acoustic performances (especially) are very emotional and well executed. They're more than fine.

Katy can sing. Her issue is that, due to her poor breath control, she can't sing and dance simultaneously and she sounds tired relatively quickly. I felt like when watching the Prismatic World Tour that she didn't really sound that great until the second segment, almost as if it takes her a while to settle into a performance, something that she doesn't get a chance to do over the course of a 4-minute song. She has visibly grown though - watching some of her OotB-era performances or 2010 performances of Firework are somewhat uncomfortable but she's shown time and again that she can do those songs justice these days.

TL;DR: Katy can sing, so yeah.