Distributed denial of service (DDoS) attacks typically rely on having a group of thousands of computers all sending floods of traffic to a victim.
Since the goal is to overpower the bandwidth of the victim, they typically send UDP packets since those don't require handshakes or coordination.
But because Tor only transports correctly formed TCP streams, not all IP packets, you cannot send UDP packets over Tor.
(You can't do specialized forms of this attack like SYN flooding either.)
So ordinary DDoS attacks are not possible over Tor. Tor also doesn't allow bandwidth amplification attacks against external sites:
you need to send in a byte for every byte that the Tor network will send to your destination.
So in general, attackers who control enough bandwidth to launch an effective DDoS attack can do it just fine without Tor.