To add to this:
A chargeback is for when you do not receive goods or services that you paid for.
There’s two transactions here.
Your Monzo account → PayPal (a Mastercard transaction, which chargebacks are available)
PayPal → scammer (if “Paying for goods or service” is chosen, some protection is availabe).
The part Monzo is involved in all happened as expected. PayPal received your money, correct? That is what is meant to happen in that transaction. If you were to say “hey, I sent money to PayPal and have been charged, but my PayPal balance didn’t topup” then sure, that transaction was a problem and raise a dispute.
The problem transaction here though, is the PayPal → scammer part. PayPal has sent money to the scammer (on your instructions). It has nothing to do with Monzo, frankly.