According to Catholicism, do evil spirits need bodies to manifest themselves to people?
The answer is NO!
A good example is when Satan tempted Jesus Christ in the desert;
Matthew 4:1-11 New International Version (NIV)
Jesus Is Tested in the Wilderness
4 Then Jesus was led by the Spirit into the wilderness to be tempted[a] by the devil. 2 After fasting forty days and forty nights, he was hungry. 3 The tempter came to him and said, “If you are the Son of God, tell these stones to become bread.”
4 Jesus answered, “It is written: ‘Man shall not live on bread alone, but on every word that comes from the mouth of God.’[b]”
5 Then the devil took him to the holy city and had him stand on the highest point of the temple. 6 “If you are the Son of God,” he said, “throw yourself down. For it is written:
“‘He will command his angels concerning you,
and they will lift you up in their hands,
so that you will not strike your foot against a stone.’[c]”
7 Jesus answered him, “It is also written: ‘Do not put the Lord your God to the test.’[d]”
8 Again, the devil took him to a very high mountain and showed him all the kingdoms of the world and their splendor. 9 “All this I will give you,” he said, “if you will bow down and worship me.”
10 Jesus said to him, “Away from me, Satan! For it is written: ‘Worship the Lord your God, and serve him only.’[e]”
11 Then the devil left him, and angels came and attended him.
We can see from above gospel narration that Satan did not indwelt any human bodies to manifest and communicate himself to Jesus Christ the Son of Man.
A lot of examples can be found in the Mystics of the Church wherein demons manifest themselves in their horrible form to some victim souls. A good one is St. Gemma Galgani.
See this link;
MYSTICS OF THE CHURCH
THIS WEBSITE IS DEVOTED TO THE EXTRAORDINARY MYSTICS AND VISIONARIES OF THE CHURCH, ESPECIALLY THOSE WHO ARE LESSER KNOWN, SUCH AS ST GEMMA GALGANI, BLESSED ALEXANDRINA DA COSTA, SR CONSOLATA BETRONE, THERESE NEUMANN, REV. PERE LAMY, GABRIELLE BOSSIS, JOSEFA MENENDEZ, MARTHE ROBIN, SERVANT OF GOD LOUISE LATEAU, BLESSED ANNA MARIA TAIGI, SISTER MARY OF THE HOLY TRINITY, SISTER MARIA ANTONIA AND OTHERS. THE AUTHOR ENDEAVORS ALWAYS TO BE IN COMMUNION WITH THE CATHOLIC CHURCH AND ITS TEACHINGS.
The only mysterious question is, in the Garden of Eden why would Satan opted to indwelt the serpent if he can manifest himself even without a body?
The answer is, Satan had opted to hide from the serpent because appearing as a fallen angel will find himself inferior to Adam & Eve who were both cloth with the Light of God.
Satan if he opted to appear in his angelic state as cast out, will only have a "false light".
"And no wonder, for Satan himself masquerades as an angel of light." (2Corinthian11:14)