No, "more clear" is not a mistake.
Usually, one would probably choose "clearer" for the adjective. But some speakers and writers choose "more clear" because it sounds better in a particular sentence or because it is easier to hear than "clearer."
If you will type "more clear" in the "books" section of Google, you will find that many good authors have used "more clear" in very serious writing.
A similar case is "more polite" and "politer."