IMHO, sandals are fine for poor, itinerant preachers such as Jesus Himself or Franciscans or Dominicans. However, if you are an ordinary, worldly member of the laity and you are not homeless or destitute, you probably own a set of dressy clothes that you may wear to work or funerals or something. These are the clothes that you should be wearing to Mass.
I have a very low income and I do not spend money on clothes for the fun of it. I tend to pick up my dressier things at thrift stores nearby. But I maintain a "business casual" style of dress for attending liturgies, and I feel that this is the minimum necessary for respectful attire at my suburban parish. Our pastor is an orthodox man and requires strict decorum, especially from his ministers and volunteers, of which I am one in service to the choir or as an altar server.
I would not dare show up as an altar server in anything less than my button-down, collared white shirt, black dress pants with belt, and black dress shoes. Father has been known to turn away would-be servers who do not meet the dress code. Likewise, he has had some stern talks with the choir director about members wearing proper clothing. We live in an area where it gets extremely hot, and men and women alike must be reminded each summer of the importance of decorum and modesty in dress. A section in the bulletin is dedicated to this purpose during the hot months.
I do not know what kind of pants and shirt you would wear with the sandals in the OP, but they would seem rather out of place in a "business casual" or better ensemble.
Just as I do not consider myself eligible to wear a Franciscan habit, I do not consider myself eligible to wear Franciscan sandals to Mass. Your mileage may vary!