There are a wide array of different modes of thinking when it comes to eating fruit. Some people who are bodybuilders believe that fruit should not be consumed at all or in very small quantities. Their argument is that fruit is filled with fructose and that can be harmful for fat loss and body composition. While this is true, it is not something that you really have to be all that worried about in the long term. However, if you are eating a fruit smoothie, this is something that can get taxing on the body really fast.
While the fruits have plenty of nutrients, they also have a lot of sugar and this is bad for the body. The immune system will start to see some negative downturns with too much sugar and there are problems with fat gain and insulin as well. If you are trying to lose weight on a diet, getting too much sugar in your smoothie could be the last thing that you need. Instead, you might want to add some nuts and spinach which will not really alter the flavor of a smoothie (especially if it already has some sweet fruit in it), but it will make it a lot healthier.
Most of the time a fruit smoothie is not going to be bad for you unless you are eating it with a lot of different high sugar fruits and you are eating them along with a lot of other sugary foods. If it is the only sugar that you are eating for the day, it could not be a big problem. The other good thing is that fruit comes with a lot of fiber, water, and nutrients so it is definitely better than just eating a cookie to get all that sugar!
At the end of the day, you need to be careful with your sugar consumption and sometimes fruit smoothies can go overboard.