Is overnight parking allowed here?

There are many Walmart stores that actually encourage overnight parking. In fact they used to advertise it as a benefit. The bad part is that a lot of bad things happen at a few of them occasionally

There are security patrols and a police surveillance pole with cameras. I wouldn't advise it.

I doubt overnight, however you can always ask management since they have more flexibility. They have security that circles the parking lot day and night, and they will tow you otherwise.