He was excessively nice to the point where it became annoying.
overly nice
Frequency:
70
%
She was being overly nice to cover up her true feelings.
unnaturally nice
Frequency:
60
%
Her unnaturally nice demeanor made her seem insincere.
sickeningly nice
Frequency:
45
%
His sickeningly nice behavior made everyone suspicious of his motives.
Idiomatic Expressions for overnice
Phrase: Overnice manners
Meaning: Excessive or affected politeness
Usage: His overnice manners made some people uncomfortable as they found it insincere.