I don't think it looks bad but a 22.5 miter or round over would be simplest. The best looking is a cove on end of baseboard that fits curve on casing.

Good idea. Thanks. Now that I look at the photo again I see that the casing needs to be extended too.

Typically, casings are thicker than base mouldings in order to avoid the problem you have. You can modifiy the casing with a plinth block or a back band, but those will alter the style--and probably not in a pleasing way. You could also chamfer the front vertical edge of the base.

I recommend you return the base and purchase base of a thickness that's less than or equal to the casing.

Oversight in selection of base. Looks like PVC, no need for that when the other millwork doesn't have any. As C Ross Homes said.