DDoS to blizzard server: possible but costs a lot to do it.
DDoS to another player in game: theoretically it is impossible. the other players IP address is not revealed to your client. All your connection/communication is to blizzard server. Unless there is loophole on server.
DDoS to your ISP server or backbone server of Internet: It happened a lot. But it is not Game related issue.
Other thoughts: sometimes the game server on Blizzard side would drop some players due to complicated reasons. The server is not down, just drops the connection to certain IP range addresses. it rarely happened but it does happen.