I don't see the Orville as Star Trek fanfic. Certainly influenced by, and a sort of an homage to the original show, to be sure.
And yes, it's rather cheesy. But it's cheesy on purpose, not by accident or due to any failing on the part of the writers and cast.
And let's not forget, the original Star Trek was for the most part rather silly when you come right down to it – terrible self-involved acting by William Shatner, unsubtle plots with unsubtle points, and of course, the famous papier-mâché rocks. And yet we all, or at least many of us, loved it.
I see the Orville as a modern day version. Everyone seems to be judging it as if it were a failed attempt at a serious space drama. It's not; it is what it is. So it all comes down to a matter of taste – most critics and apparently most viewers do not care for it. I rather like it, and thoroughly enjoy it as well. I doubt that it will last; when both critics and viewers pan something, it usually doesn't long for the world.
Which is a shame – there's really no other show like it on TV these days. And, in fact, when it's gone I predict it will gather somewhat of a cult following, much like Star Trek did – let's also not forget that Star Trek did not become the iconic show as we see it today until it was taken off the air having been deemed as unworthy to continue.