Emma certainly never treated Miss Taylor as a servant. A governess was not exactly a servant, even in that era, it was the only profession opend to gentlewpomen, but many proud and inconsiderate employers often treated their governesses like servants. A 19th century governess often occupied a very lonely position -- she could not mix freely with the servants because she was "above" them, and could not associate freely with the family becasue she was "beneath" them.

In this case, however, Mr Woodhouse was a gentle, kindly man, and his daughters had lost their mother and needed a mother figure --
"Her mother had died too long ago for her to have more than an indistinct remembrance of her caresses, and her place had been supplied by an excellent woman as governess, who had fallen little short of a mother in affection."

And in chap 5, when Mrs Weston disagreed with Mr Knightley about the dangers of Emma's intimacy with harriet, Mrs Weston said --
"I know that you all love her really too well to be unjust or unkind; but excuse me, Mr. Knightley, if I take the liberty (I consider myself, you know, as having somewhat of the privilege of speech that Emma's mother might have had) the liberty of hinting that I do not think any possible good can arise from Harriet Smith's intimacy being made a matter of much discussion among you."

Miss Taylor was a mother figure for Emma.