I know you can sometimes spontaneously create them when performing alchemy if you have the skills, and I'm constantly picking up lesser variants off of dead enemies. However, since mutagens are not reverseable or upgradeable once they've been applied, is there any actual use to the lesser mutagens? It would be nice, and also fit into the alchemy system if you could somehow combine lesser mutagens into higher level ones but I certainly have not seen this ability anywhere in the game?
Nope. Lesser Mutagens do have the advantage of being very good vendor fodder for their weight (~9 orens / .1 weight ).
If you (smartly) want to wait until you have greater version of whatever mutagen you're looking for, the only use the non-greater mutagens are good for is selling them.