It can be, but unless is a mass theft or there's a big media influence on it... it's unlikely it's a case which would be prosecuted. It's up to the prosecutor to move forward with a criminal case and flowers at a grave isn't usually worth the expense of the prosecution.
While people do steal from graves, a lot of things missing from a grave are because the cemetery itself actually removed and discarded them for a number of reasons.
A lot of it is maintenance of the property (like cutting the grass is an example) and removing things that are in the way. Some cemeteries have strict guidelines of what can and cannot be placed on a grave site. The cemetery my dad is at is quite particular of what can be on graves things like figurines, stuffed animals, toys, shepherd hooks, solar lights and things like that are in the "cannot be placed" category and the cemetery will remove them. There's also a schedule of when certain items can be put down and when they need to be removed by. Grave blankets as an example can't be put down til late November and have to be removed by March 1st, anything not removed the cemetery discards.
There's a little boy buried next to my dad and his grave was always really elaborately decorated, they had a shepherds hook with a soccer ball and the cemetery took them down several times.
I actually had to sue the cemetery my dad is buried at. He has a bench style headstone and they hit it when they were digging a grave with a backhoe. It badly damaged and cracked it. They refused to fix or replace it because I didn't buy it from them. Those are not cheap by any means. I spent almost $4000 on it. $3200 for the actual bench and then $700 to the cemetery for their installation fee. I worked a lot of overtime to pay for that. So I took them to small claims court and I won. Told the judge I either wanted the price of the bench back or for them to replace it (they also sell them, but they cost a whole lot more through them) and we settled on them replacing it.