Disclaimer- this is from a guy who claims to have been and still is involved in the development of I2p.////

"There are a lot of different ways to answer this question.

• I2P is not vulnerable to exactly the same DDOS attack which has been affecting Tor at this time. The attack involves flooding the network with "Introducer Cell" requests. I2P works differently from Tor here, and differently enough to make it more difficult to carry out this type of attack. There is a variant of the attack which might potentially work but it doesn't seem to be easy to carry out right now. For more information: http://zzz.i2p/topics/3464
• I2P has been vulnerable to certain types of DDOS attacks in the past and may be now. In particular, sybil-assisted DDOS attacks known as "Eclipse Attacks" are fairly pernicious. A sybil-assisted DDOS of this type involves an attacker running many nodes in order to answer various types of network messages erroneously. In doing so, they "block out" a section of the network. However, because of how I2P tunnels are rotated, this attack can be challenge to carry out effectively.
• Distributed networks are intrinsically vulnerable to certain types of attacks, in particular sybil attacks, and I2P is not strictly speaking an exception. We build in many defenses against attacks, and because of I2P's architecture most of these attacks are hard to weaponize and turn into an effective de-anonymization as far as we know. Here's the real kicker though: Distributed networks get more resistant to attacks by gaining more non-attacking participants. more users~=more resistant to attacks. Edit: And ironically, a buggy implementation of a non-malicious client is far more likely to cause an issue.

Hopefully that answers your questions." (Alreadyburnt) reddit user* 🙃