Probably yes. For billboards, viewing distance matters more than the physical print size. A very large print seen from far away does not need the same pixel density as a small print viewed up close, so the resolution difference between consumer and pro DSLRs is usually not the deciding factor.

For a 20×15 ft billboard viewed from roughly 35 ft or more, most modern DSLR files are often adequate. The better test is practical: print the image at high quality, view it from a proportionally similar distance, and judge whether it looks detailed enough.

If the billboard must hold up to close inspection, even many pro DSLRs may not be enough on their own. In that case, shoot multiple overlapping frames and stitch them, or use medium/large format equipment.

So: don’t focus only on megapixels. A consumer DSLR can be good enough, but success depends on viewing distance, image quality, technique, and whether the final use demands unusually high detail.