Illegal means criminal in some jurisdiction.

Very unlikely there's any issue of legality (criminal offense) related to any iframing.

More likely you'll be breaking some TOS (Terms of Service) + if the site catches you, they'll block you breaking their TOS + ban you.

This becomes complex if you try breaking tech related to payments like Stripe or PayPal, because if you're banned they may retain + refund whatever money they've collected for you.

They may also, reverse payments, sucking all the money they can out of your bank account to pay back historical (older) purchases.

The real answer relates to your jurisdiction of citizenship + the actual site your violating.

If it's a money related site - bank, brokerage firm, etc - likely you may end up in a heap of trouble.

If not, likely you can violate however you like, till your caught... then setup another account + start violating again.

If you take this last approach, likely best to use a VPN or VPS pass through, as some sites will block your IP for a long time also.