You are really not a mean mum, not even close!
Vegetables should be eaten at least once a day, no exception. This was what I knew, in a recent post here from a person of a nutrition course (www.mumsnet.com/Talk/teenagers/3840892-Can-I-ask-a-cheeky-question) I asked "for teens, how many meals in a day or week should include vegetables?" The answer: Honestly? All of them.
And for what I've seen that's right. I already include vegetables in the majority of meals at home but I'm going to include them in every possible meal, very close to all of them.
I would present vegetables as a food you eat mainly for healthy reasons, it's not really a matter of pleasure and taste. I'm not a fan of the "clean your plate" club as a general rule, but veggies need to be eaten, period. Frankly the "vegetables battle" is something more of young kids than teens, if your children want to be mature they should not complain and just eat them. If the mom still needs to enforce a healthy diet (totally right, it's just good parenting), the mom is not the one who is mean, they are the ones who are acting as younger kids!
As side note, in addition to the meals, you could make for them a lot of smoothies with vegetables and fruit, mixed in a free and creative way, very easy to be taken, quickly even if they are in a hurry.
Don't give up (if not, add some more veg)!
For personal experience I can say they will definitely benefit of it and they will thank you later.