It is small wonder that she was so upset. 《牛津》

No wonder advertisers are running away.  （COCA语料库）

It's no wonder you're hungry; you didn't have any breakfast. 《韦氏高阶》

Small wonder that we're lost, since we didn't ask for directions. 《韦氏高阶》

No wonder you couldn't open the door – it was locked! 《The Free Dictionary》

No wonder you're tired, you've been walking for hours.《牛津》

从词典释义以及以上词典例句，你可以知道 no wonder后接主谓结构实际是省略了 It is。无论是否省略 It is，它都是主句。因为如果后面再接句子，二句之间可以用分号，后面句子可以用since引出原因状语从句，也可以用破折号表示后面句子为补充语（补充说明主句）。所以，如果二个句子之间用了逗号，也应该把逗号后的句子按照补充语理解，逗号与破折号的作用相同，只是没有破折号强烈而已。因此，不应把no wonder分析为连词，no wonder+主谓结构应该分析为主句，而不是从句。