Yes, you can (and should!) store at least one backup of your wallet.dat file (assuming Bitcoin Core wallet) on a flash drive. More backups are good, so long as you keep them in a safe place where they can’t be found by others.
No, it’s definitely not as safe as a Ledger or any other hardware wallet. Anyone who takes or copies the flash drive can access the wallet and steal all your bitcoins. This is not the case with a hardware wallet, which is protected by a PIN (and optional password).
Even if you encrypt the USB drive, it’s still not as good. Hardware wallets will protect your bitcoins even if your system gets infected with malware, this is not the case with a USB drive.