Gaga has made a pretty good impact imo. and yes she did kind of bring back dance into the mainstream. not to say it was completely eradicated, but it was a rarity to have a dance hit that charted high and played much on the radio. Gaga sort of brought that back, with help from surrounding artists who had a similar drive like the Black Eyed Peas and Katy Perry.
but for fashion and social reform, she has definitely made an impact. i also like to think she instigated the trend music is taking to using alternative music and instruments in songs