Sure it is. Article I of the Constitution says that all legislative power shall be vested in Congress. That means anything that has the force of law--and all Federal regulations created by executive branch agencies under the current US regulatory regime have the force of law; you can be fined or jailed for violating them--has to be passed by Congress using the process described in Article I. So any Federal regulation that has not been passed by that process--i.e., every one of them--is unconstitutional.

The fact that current US jurisprudence disagrees with that statement just illustrates how far current US jurisprudence has diverged from what the Constitution actually says. The status of Federal regulations is by no means the only example: current US jurisprudence says that Congress can regulate farmers growing crops for their own personal use because of the Commerce Clause; and that a city government can use the eminent domain power to evict people from their homes and turn the property over to a private development corporation (that ends up never developing the land anyway), and that counts as a "public use" under the Fifth Amendment.