Answer:
Depends on whose side of the story. Enki killed Tiamat's husband Apsu. Tiamat was willing to forgive her son of this transgression. Depending on who was telling the story, Apsu's anger at Enki and the younger gods was either justified or not. Enki set the younger gods to doing tasks. It is Tiamat who carries the tears of the younger gods to Enki. It is she who encourages Enki to create humans. The disagreement between Enki and Tiamat is if the gods would share the burden with their new helpers the humans. Enki told his mother that humans would shoulder the burden of the gods. Since Kingu name means the unskilled worker, I assume that the battle between Tiamat's and Enki's forces is symbolic of an upper versus lower class struggle. Note that in early sumerian religion, that Tiamat is called Nammu and is not referred to as evil. Tiamat is angered by Enki's answer and declare war. Note in her sorrow for both the murder of her husband and the enslavement of mankind, she give birth to the demons that torment mankind. In later legends, humans are not created until after her death by the blood of Kingu. Tiamat represent the alliance of sealand which was destroyed by the followers of Marduk.
Now, Tiamat was in existence before the birth of either Utu (Shamash) or Marduk, so she is a goddess of darkness. Since she is a compassionate goddess, in forgiving her son or worrying about the suffering of the humans and the lesser gods, I would not classify her as evil.
If you are talking about the dungeons and dragons character and not the character from the Enuma Elish, then the answer is yes.