The 'er' ending makes it feel masculine for me. So I agree that it would need a feminine-sounding middle name to work on a girl.
Thayer
AryaThayer
AtalantaThayer
Cassandra Thayer Catelyn
Thayer
Edith Thayer
HelenThayer
Laura Thayer
LaviniaThayer
MariaThayer
MorriganThayer
NatashaThayer
SabinaThayer
Sonya Thayer
Velvet Thayer
Wanda Thayer
Zula