The draft is written well and easy to follow the proposed extensions for those who are familiar with the Babel Protocol as described in RFC 8966.

In my view,  a couple of minor updates would read better for the novice reader.

In the Introduction, section 1, last but one paragraph -
--
 We call a route towards an IPv4 prefix that uses an IPv6 next hop a
   "v4-via-v6" route.  This document describes an extension that allows
   the Babel routing protocol [RFC8966] to announce v4-via-v6 routes
   across interfaces that have no IPv4 addresses assigned 
(suggested append..)
"and are capable of forwarding IPv4 traffic over non-IPv4 interfaces".
(end of suggestion - or something similar to that effect..).
--
On support for ICMPv4 packet generation by Babel node that has IPv6 interfaces..
While this topic is covered, it would be nice if its importance is mentioned somewhere in introduction
or in section 3. 

Such as -
"For hop-by-hop reachability distribution, it is important to facilitate 
ICMPv4 "destination unreachable" from non-IPv4 Babel router to notify the source about incomplete IPv4 path (i.e. dangling IPv4 path)."

Again these are suggestions to improve readability and author(s) can provide the right text if applicable.

I must admit that I am not intimate with Babel protocol so take above suggestions with a pinch of salt..