可以，当形容词的时候都是胆小的没区别，
adj.
懦弱的，胆怯的
表现恐惧的；由于恐惧引起的
以上来源于：《21世纪大英汉词典》
timid
英 ['t?m?d] 美 ['t?m?d]
adj. 胆小的；羞怯的
[ 比较级 timider 最高级 timidest ]
cowardly
英 ['ka?dl?] 美
adj. 怯懦的，懦弱的；胆小的
adv. 胆怯地
柯林斯英汉双解大词典
cowardly /?ka?dl?/ TEM4
1.
ADJ If you describe someone as cowardly, you disapprove of them because they are easily frightened and avoid doing dangerous and difficult things. 胆小的; 怯懦的[表不满]
例：
I was too cowardly to complain.
我胆小得都不敢抱怨。