Yes absolutely please do!
Feeding stray animals will increase their population beyond the capacity of the surrounding environment, and therefore overhunting their preys. You're just killing wild birds unnecessarily.
Plus it will make the stray cats less afraid of humans. Humans are the most dangerous creatures ever. The fact that one human feeds them doesn't make it otherwise. Learning to avoid humans is generally good for them.
That said, probably still best to get them indoors where they don't have to worry about being hit by cars or attacked by other animals or any of the other harms in the wild... although it's hard to say if every cat would choose to trade sunshine and freedom for the extra safety and comfort. They are barely domesticated animals that evolved to live outside after all.
If by "get fixed" you mean to neuter them then maybe. But most people who feed feral dogs/cats are just feeding them because... because they want, or their gut feeling says it's the right thing to do.
I'm not saying if you feed wild cats you're literally Satan. But feeding wild animals needs responsibility. Actually it takes MORE responsibility than just having a house pet, because now your actions are affecting multiple lives instead of one.
They absolutely decimate song birds populations.
I spent a lot of time a few years ago helping people trap and neuter their feral colonies, keeping the population under control with the support of the caretakers rather than fighting them.
Unfortunately, this won't solve the problem of feral cats, it only mitigates it. Culling doesn't really seem to work either though. Chemical sterilization is a promising approach, but hard to enact on large scale without sterilizing other species by accident.