A subtweet is such a common thing that it has entered the English lexicon. The term itself has a very specific meaning according to Oxford Living Dictionaries:
(on the social media application Twitter) a post that refers to a particular user without directly mentioning them, typically as a form of furtive mockery or criticism.
‘while he didn’t include Smith’s Twitter handle, that didn’t stop Smith from seeing the post, taking umbrage, and firing off a subtweet of his own’
Subtweets are one way to provide feedback on a hot button issue without becoming the target of harassment by those directly involved in the issue.
It’s also a way to passive-aggressively escalate an interpersonal issue. This is done in unhealthy ways across many communities. Remember, like most tools, a subtweet can be used to great benefit or great detriment.
“Switching communication venues is an escalation. —Nathen Harvey“
But, even the Washington Post believes subtweets are terrible, Study confirms what you always knew: People who subtweet are terrible. Thus, the general impression is a subtweet is a very negative thing.