Yup - this is the kind of thing you could be feeding your guinea pig
every day!
It is vitally important for their health that they get a good portion of vitamin c rich veggies daily. Along with the lettuce mix (which is basically what your mix is), you need to offer bell pepper, coriander/cilantro, courgette/zuccini, a baby carrot, cherry tomato, romaine leaves, cucumber and lots of other veg.
Here is a good link to other veggie ideas.
Do you have good quality hay available constantly too? This is just as important as the daily veg and water!