In your specific example? Yes.
I think there are plenty of reasons that flak-like languages should be tagged with brain-flak
Miniflak is a direct subset of brain-flak, so you could even get away with pretending the language doesn't exist and calling it a brain-flak restricted-source question.
Brain-flak Classic is brain-flak, just less evolved/refined. Classic was the first finished version of brain-flak to be used/published. The current version is the original with the
 functions changed to make it more powerful. As far as I'm concerned, classic is like a version zero, and the current is version one. FWIW, the only challenge specific to classic was written before the change. I noticed this a while ago, and edited the title/description to avoid confusing people about what
 is supposed to do.
I highly doubt there will be enough questions specific to a variant of brain-flak to make it worth adding a tag for any variant.
But there's a more general question too. "Should questions about X be tagged with Y, when X is inspired by/a subset of Y?" This is a good question since we have tons of language-variants. For example, there is V/Vim, J/Jelly, Python/Pyth, Actually/Seriously, Brachylog/Prolog, etc. Not to mention the 8-trillion brainf*** variants.
In this more general case? It varies.
In a lot of cases, it comes down to discretion, but a good general guideline is
Is X largely backwards compatible with Y, or Y with some gag tacked on? If so, the y tag is a appropriate. Otherwise, it should be untagged or have it's own tag made.
For some more concrete examples:
If I were to ever make a V specific question, I would most definitely tag it with vim. This is because V is mostly backwards compatible. (If I had to guess, I'd say ~90%~ of vim programs could be run in the V interpreter with no problems) Also, I don't think it's worth creating a tag just for V.
Questions about Ook! or compressedf***, or bitf*** or whatever variant of brainf*** you have should be tagged with brainfuck, because in most cases they are the same language, with the silly substitution rules tacked on to the end.
Questions about Pyth or Jelly or Brachylog should definitely not be tagged with python or j or prolog respectively. This is because they have evolved enough to pass the point of being "a variant", and as far as I know, attempting to run J code in the Jelly interpreter or vice-versa would absolutely not work. Same for the other languages. (I don't actually know how to use any of these languages, so someone please correct me if I'm wrong)
I purposefully said "Untagged or have it's own tag made", because this varies too, mostly depending on popularity of the language/number of potential questions to tag it with. I'd say if you can foresee putting the tag on at least 3-4 questions, making a new tag should be fine.