It would still take a long time to train Ukrainian pilots and ground crew to operate the US jets. I'm no expert, but I've read some things written by former US pilots that say its not so easy to jump in and take off. All of the dials, gauges, markings in the cockpit are in English not Ukrainian. Old jets need parts and expertise to keep them running.Only problem is that I think most of our allies that had MIG's are running out of them because they where mostly leftovers from when they got their military hardware from Russia.
So it's not like there are a bunch of MIG's to go around.
On the other hand we have thousands of aircraft that have aged out or are going to age out very quickly that are still superior to anything the Russians are flying. And that's just our junk aircraft that we don't even really need anymore. That's to say nothing about the modern aircraft that our MIC is able to produce.
Honestly since the military is adopting a new battle rifle, why not just ship all our old M4's and M16's out to Ukraine.
Rifles are simpler. IF we really have a surplus, then yeah, maybe send those.