For other non-german speakers:
If you translate „Das Gelbe vom Ei“ word by word it means „the yellow of the egg“ i.e. the interesting/ the important part. If something is „Nicht (not) das Gelbe vom Ei“, it means something between „not as good as expected“ and „a bit shitty“. (imho his range does include „nothing to write home about“)
For other non-german speakers: If you translate „Das Gelbe vom Ei“ word by word it means „the yellow of the egg“ i.e. the interesting/ the important part. If something is „Nicht (not) das Gelbe vom Ei“, it means something between „not as good as expected“ and „a bit shitty“. (imho his range does include „nothing to write home about“)