Omni plants when planted by buydebug are always Normal quality, but there are 2 ways you can improve their quality once planted:
1) Use NRaas Debug Enabler to set the quality higher/lower
2) Purchase the Swiftgro Gardening Station* from the Sims 3 Store and nag your plants to death with it, they will randomly bump up in quality all the way to Perfect (mine would skip Nice quality which might be due to being a plant sim with level 10 gardening, green thumb trait, and super green thumb LTW.)
It's also possible you could talk an omni plant to death to make it increase its quality since it works using the Switftgro store item but I haven't tried doing that myself.
If you attempt to clone them using the dropper tool and testing cheats on you'll always end up with a Normal quality (this was only with the buydebug plants, don't think you can clone the ones that come in the post), and if you do it using NRaas it doesn't work (I even attempted to use NRaas to add everything to the family inventory in case the clone was just displaced but it didn't exist.)
(This one's for ejnarts1) You'll find that when selling back produce whether made from an omni plant or grown from a seed or plant that they will sell back at a value that's only given to items that are home grown. For example if you buy an apple from the store and sell it back you'll get $6 for it, but if you sell the same quality apple that you grew at home you'll only get $4 for it. This could be why your book was worth less than one that wasn't reproduced by the omni plant.
This is the only topic in relation to what I'm posting, so yes I know it's 3 years old but it's relevant and answers some questions that are in here.
*btw a bit of a warning about the Swiftgro Gardening Station, it affects plants further than the footprint guide when placing one. I'm not sure exactly how far but it seems to be 2 squares left and right and I think 4 squares above its placement, so if you are aiming to make different quality produce don't place one too close to your other plants.