Is there a halachic problem with wearing
a) designer clothes with writing e.g. “Hollister”
b) denim skirts?
a) no (though it shouldn’t be ‘attention-grabbing’).
b) no (if it isn’t tight).
However, note that there can be sociological issues involved, and this is also part of “halacha” in the sense that if the community sees items of clothing as being immodest, they should not be worn even if they are not halachically problematic.
This depends on the norms of the community in which you live.