There is not a general way of saying "someone is acting something" in French. It depends on the sentence, on the context, etc. In your sentence, you could translate as "Elle donne toujours l'impression d'être faible et fragile."
Other examples:
she just acts dumb --> elle fait l'innocente
you acted like a fool --> tu t'es conduit comme un imbécile
she's just acting like she's angry --> elle fait celle qui est en colère
As you can see, there's plenty of ways of saying it.