Does 告 posess negative connotation?
but it appears to mean the sound a cow makes.


It seems wrong. See here:


  牛觸人，角箸橫木，所以告人也。
  如許說則告卽楅衡也。


[ The cow butts people, with ledger on the cow horn, so means 告人 (report to people).
As 许 said it is 楅衡 (the ledger tied on the cow horn to avoid the cow butting people. ]

Anyway, it seems to have nothing to do with "the sound a cow makes". So (also based on the sense of a native Chinese)


  Now I wonder does 告 still posess some of it's etymologycal
  connotation?


If you mean "the sound a cow makes", no.


  Does 告(to tell) sound vulgar to native speaker?


No, basically it's neutral.


  Are there words to insult someone using "Cow talking" analogy?


No.


  Does it have the connotation of "to fink" (to be an informator)?


Yes. Such as 告发, 密告, 告状.


  Or maybe it has something to do with LOUD talking just like cows
  always moo'ing loud?


No.