Leading Dublin hairdresser David Marshall said Kathryn was too old to carry off the tight corkscrew curls and should swap it instead for a more mature style.
"It did nothing for her, it was too young for her and more of a teenage look," Mr Marshall told the Herald.
He also said Kathryn's hair was too frizzy.
"The centre parting is not good for her. Also it looks a little frizzy and needs a good conditioning treatment," the hairdresser explained.
The blonde Carlow native's hairstyle was also the focus of much discussion on Twitter.
Many viewers likened her fuzzy style to child star Shirley Temple gone wrong.
Others compared the curly do to a poodle, fusilli pasta and even an Eighties-style perm.
One comment read: "The hack of Kathryn Thomas's hair! She's like Shirley Temple gone wrong!"
Another said: "Katherine Thomas's hair is vile this week, worse than any other week! Looks like she got electrocuted!! Looks like fusilli pasta!"
Top Dublin stylist Kevin Barry also felt that Kathryn was too adventurous with her "young" do.
But hairdresser to the stars, Dylan Bradshaw, leapt to the 34-year-old's defence and commended the presenter for trying out different styles each week.
"I think Kathryn wears curls well and they are always quite nice on her," Mr Bradshaw told the Herald.
One Voice star who did impress stylist Kevin Barry with her long, luscious locks was RnB singer Jamelia.
"It looked full of life and in great condition," Mr Barry added.
But Mr Barry, who is manager of the David Marshall hair salon on Fade Street, felt that Sharon Corr's curly blow-dry for Sunday's show was "very plain and boring for such a beautiful girl".
The Voice of Ireland live final will take place on RTE One this Sunday and the winner will walk away with a lucrative record deal with Universal Music worth €100,000.