As long as you regulate the output of the solar panel down to 5V then it should be doable. If the power bank supports USB PD charging you can get a USB PD module or car charger that takes 12V in, regulate the panel voltage down to 12V and power the PD module.
That said, solar panels are not going to provide consistent voltage due to clouds and shadows and such. That’s why pretty much every solar system has at least a small battery to buffer the output of the panels. Since you’re just charging a power bank it’s probably not a big deal.
Also, any large panel will be completely wasted charging a USB power bank, so you should just get a small 10-15W or so panel. A power bank won’t charge much higher than that at 5V.