No, you should not be worried, but you should be checking GitHub's server fingerprint rather then relying on IP addresses.
git command you do it only on first (ever) connection. If the public key (corresponding to the fingerprint) is in
known_hosts file, subsequent connections to
github.com cause only warning if the domain got resolved to a new IP address.
It is equivalent of using
ssh command with
You should be cautious (and check with a trusted source) if a new fingerprint was displayed on your screen for confirmation.
ip-lookup.net figures out what DNS name is associated with an IP address using a reverse DNS.
In case of
126.96.36.199 it queries the address
188.8.131.52.in-addr.arpa which does not have a corresponding forward DNS record.
It seems like a misconfiguration or not strictly enforced-one on GitHub side, however the verification mechanism would not be viable in this scenario.
In case of Git (SSH) client connecting to
github.com domain, if you assumed a compromised DNS server redirecting to a rogue IP, the verification of the destination address using reverse DNS lookup would be susceptible to the same attack (compromised DNS server would confirm the address to belong to
github.com owner with reverse DNS).
Regardless of IP ownership verification, checking the server fingerprint assures no server impersonation or MitM attack is taking place.