Yes, you can cook vegetables too long which would make them mushy. This is more likely to happen with produce like broccoli and asparagus. Carrots (along with potatoes, cauliflower and other denser vegetables) can take more time to cook all the way through and are more suited to longer slow cooker times (3-4 hours on high or 6-7 on low). Sometimes it will be based on personal preference - if you like soft carrots, cook them a little longer, if you prefer more firm carrots, cook them a little less.