Whenever I feed a pet I get various comments like "yum" and "that is gross" which seem to have no real relation to whether the food actually seems gross. Just wondering, is it something to do with species, hunger, something else, or completely random?

It's completely random. It doesn't even stay the same if you were to feed the same pet the same food twice.

also no pet ever refuses any food if it is at all hungry, no matter what her reaction was last time. Unless the pet says she is "too full to eat that", it gets eaten. If it's a new item your pet hasn't eaten before, it then gets added to that pet's "foods eaten" list.

Toys work the same way, except there's a no "toys played with" counter or achievement.

I did try feeding the same pet the same item twice, thought the comment might have something to do with the hunger level. And they seem to have no problem eating a food they called gross or having that food show up on the foods eaten list...

"foods eaten", this is only for Achievements or roleplaying (you could say your pet hates broccoli so badly that she never really eats it and point to the foods eaten list to "prove" this even though we all know in ...RL ... subetapets'll eat anything.)