“The show was one of the dullest we have ever seen” – tense consistency

No, there is no reason to use "had" instead of "have," at least if this is the complete context.

The tenses are perfectly fine:

• The show was: Naturally, the show has finished, and was in the past. You can say "The show began (PAST) at 9:00," "The show starred (PAST) my uncle Bernard."

• However, the status of the list of shows you have seen continues into the present (I have). For example, you say "I have (PRESENT) seen Hamilton twice," even though both viewings occurred in the past.

Try flipping it around, and reading this sentence by itself:

I have/*had seen six shows on Broadway, and this was one of the dullest.

On its own, had is incorrect here, unless you're saying it is no longer one of the dullest (because you've since seen a duller one).

Saying had implies the event in question (seeing) occurred before a referenced time before than the present, using regular past tense implies before the present.

For example, in the sentence I presented above, to say "had," the six shows would have to have occurred before you've seen your seventh show (an implicit second time reference).

As presented here, there is no second time reference, so without further context, it's odd, if not strictly ungrammatical, to use "had."

Of course, there are many ways to make it grammatical to say "had seen." But all these require expansion of the context beyond what was presented.

If you said "had," it would make it sound as if you've seen another, duller show. You could use it in a dialogue like this:

What did you think of Cats?

Well, until I saw Dogs, it was one of the dullest show I had ever seen.

See here that this adds a second time reference (when you saw Dogs), or you can do it as below:

At that point in my life, I had seen six shows on Broadway...

Both are correct, but the meaning is different. Consider these two sentences:

1. It was the best meal I've ever eaten.
2. It was the best meal I'd ever eaten.

The meal in sentence 1 is better than any other meal I've eaten, before or since. The meal in sentence 2 was the best meal I had eaten up to that time; but I may have eaten better meals since.

If you say

The show was one of the dullest we had ever seen,

you are specifically referring to your judgment at the time you saw the show.