Well you guys seem to have named most of the animals I wouldn't want to morph for whatever reason. Thinking of more animals from work, I wouldn't want to morph a Locust. There are better insects you could morph that don't stand out as much and they're UGLY.
Also wouldn't want to be a male Praying Mantis, or any male insect/archnid where the females are known to kill the males. Male Praying Mantids are incapable of copulation until they are decapitated by the female.
Not forgetting any type of prey animal which lacks natural defences. I also would hate to morph anything that's blind or has very poor eye sight.
On the list of aliens in the series, I definitely wouldn't want to morph a Taxxon, Yeerk Skirt Na, Helmacron or Venber, Gedd, Iskoort, Nesk, Mak or Sstram (two alien species also invaded by the Yeerks). However, I would very much like to morph an Andalite, Hork-Bajir, Leeran, Howler, Garatron Ketran and Kafit Bird.