Can I backup my iCloud on my iPhone with a cracked screen with a Mac?

1. To backup to a computer, you need to be able to unlock the iPhone and interact with the "Trust this computer" prompt if and when it appears. If the screen breakage prevents this, then you cannot backup to a computer.
2. iCloud has nothing to do with backing up a device onto a computer, nor does it prevent it.The iPhone may backup automatically to iCloud if connected to power and wi-fi and the screen is locked for an extended period of time, like overnight, if the option was turned on on the iPhone before the screen broke.
3. Backing up is a preventive measure, it should be done regularly before disaster strikes, not after.

Your question doesn't make much sense.

You can't backup a device to iCloud by plugging it into a computer.

Backups to iCloud happen over the air... when the device (your phone in this case) is connected to wifi and power over night.

If the phone is already disabled because of the broken screen, it's too late. You need to back it up before something catastrophic happens, not after. Not to mention the fact that 50GB may not be enough to back it up anyway...

You also need to make sure you have sufficient System Storage too, after all when you backup your iPhone it needs to temporarily prepare the backup. And the iPhone you are transferring it all onto must have an adequate amount of System Storage to hold it all. But none of this will work at all if the iPhone's screen is cracked and non-functional, that's what automatic backups are for, before disaster strikes.