It seems that we are mostly talking here about **close flags** and **very low quality** flags - since these are the two types of flags which seem reasonable in the case of low quality questions (whichever interpretation of the phrase *low quality* you choose.)

Since the OPs main concern seems to be whether the flag adds work for moderators, it is worth mentioning that these types of flags in the *ideal* scenario are actually not handled by mods but by other members of the community. The [help on flags](https://mathoverflow.net/help/privileges/flag-posts) explicitly says: "Many flags are handled by other members of the site like yourself, who've earned review privileges. Close flags, Not an Answer, and Very Low Quality are primarily handled this way."

And even the flags which are handled by moderators do not get to their *mailboxes*, but I assume you were talking figuratively in the question.

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When you flag a post as low quality or should be closed, it enters a [review queue](https://meta.stackexchange.com/questions/161390/what-are-the-review-queues-and-how-do-they-work) where other users vote what happens with the post next. 

Low quality flag sends a question into [low quality posts review queue](https://mathoverflow.net/review/low-quality-posts/). For questions which enter this review queue, [2k+ users](https://mathoverflow.net/help/privileges/edit) can choose from the option Looks OK, Edit, Close (or Recommend Close). For answers the last option is Delete (or Recommend Deletion) instead. 

If you choose *should be closed* flag, the question enters [close votes review queue](https://mathoverflow.net/review/close/). In this review queue, [3k+ users](https://mathoverflow.net/help/privileges/close-questions) can choose between the options Edit, Leave Open, Close. (As already mentioned in the comment, for users with [privilege to case close vote](https://mathoverflow.net/help/privileges/close-questions), there is no actual difference between this type of flag and casting a close vote. A flag is automatically [converted to a close vote](https://math.meta.stackexchange.com/questions/21547/what-is-the-difference-between-voting-to-close-as-a-duplicate-and-flagging-as-a#21548). However, close vote pushes the question into the review queue, too.)

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For details on when review queue is finished, see here: https://meta.stackexchange.com/questions/164288/when-is-a-post-removed-from-a-review-queue A question is removed both close votes review queue and low quality posts review queue immediately after it is closed (or deleted).

Some network-wide guidelines on what posts should be flagged as VLQ can be found here: [When to use the flag “low quality”?](https://meta.stackexchange.com/q/216107) The [official descritiption](https://meta.stackexchange.com/questions/93595/is-the-very-low-quality-flag-too-ambiguous/93606#93606) of the low quality flag is: "This question/answer has severe formatting or content problems. This question/answer is unlikely to be salvageable through editing, and might need to be removed."

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I will add that for posts which are bad enough to consider low quality flag, users with sufficient reputation might consider also vote to delete. (The reputation needed is [10k for questions](https://mathoverflow.net/help/privileges/moderator-tools) and [20k for answers](https://mathoverflow.net/help/privileges/trusted-user).) Post which meet some criteria are [automatically deleted by a system after a certain time](https://mathoverflow.net/help/roomba), but sometimes it makes sense to vote to delete manually. In fact, there already have been some related discussions on this meta: https://meta.mathoverflow.net/questions/134/proposal-to-more-aggressively-delete-off-topic-questions or https://meta.mathoverflow.net/questions/2232/should-we-try-to-re-start-manual-deletions-or-is-the-situation-fine-anyway Still, I think that it might be useful to leave some time so that the feedback why the post was closed actually reaches the OP.