Does doing an undergraduate semester exchange abroad improve chances of getting into top universities?
As JeffE says: It does not really matter whether you went somewhere and where did you go; it matters much more what have you done there. In general, having an Erasmus (or similar) semester abroad is not seen much as an advantage. Having a semester abroad accompanied by a publication with a professor there -- that's a different story, because it shows that you went there and really got some experience there.

However, if you spend the semester at a university at which you later apply for studies (which seems to be your case), you can get something from solely being there: the contacts and the knowledge of the environment. For instance: There are many PhD positions funded by various projects rather than by some general scholarships. It's mostly the project leaders who decide who gets the funding, and knowing them in person etc. can help. Still, you need to do more than just sit there at the lectures for 15 weeks.