The movie Fantastic Beasts and Where to Find Them showed that any wizard with a little skill could rob a muggle bank. Newt Scamander used kid-level spells to infiltrate a muggle bank vault successfully. He used Stupefy and Alohomora to stun a security guy and unlock a highly secure vault, respectively.
There are several things a wizard can do to rob a muggle bank easily:
Apparate directly inside a vault.
Use Imperius Curse to control bank officials.
Obliviate memories just in case.
Use spells so that muggles won't notice you at all.
Go inside the bank invisibly.
Use a bigger-on-the-inside bag to store gold and muggle money.
Muggle money can be exchanged at Gringotts, or it can be used to live a wealthy lifestyle in the muggle world.
Also, I don't think it violates the International Statute of Wizarding Secrecy or any other wizard laws. If it really violates wizard laws, tell me what they are.
Why are there so many poor wizards and witches (like the Weasleys and Gaunts) in the wizarding world?
Let's say it violates some wizard laws. Why are people like Mundungus Fletcher not doing this secretly?