seem 后面可以接 like / as if / as though ,但是不能接 as 。如：
North Wales seems (like) a good place for a holiday. 北威尔士看来像是度假的好地方。（不能说：... seems as a ...）（摘自《牛津英语用法指南》第三版 507.5 节）
It seemed like a road leading into a world beyond. 它仿佛是通向另一个世界的道路。
It doesn't seem as if (though) they will find a solution to the problem. 看来好像他们没法找到解决问题的办法。
It seems as though Ken will win the race. 看来这赛跑肯好像要获胜。 (《张道真高级英语词典》p.1371)
你的句子正确与错误情况如下：
Mary seems to be a clever girl.（正确）
Mary seems a clever girl.（正确）
Mary seems as a clever girl.（错误）
Mary seems like a clever girl.（正确）