I had a similar problem, and so even tough the question is old I want to provide another answer:
The sheet tabs height is defined by the width of the GTK themes scrollbar. I guess you're using Ambience, Ambience's scrollbars are a bit smaller than usual ones, so the sheet tabs get distorted.
I use a GTK theme with tiny scrollbars (Orta), and so those sheet tabs were barely visible.
If you use another GTK theme, for example Clearlooks, it will look normal again. If you don't want to change your whole Desktop to another theme, you can start libreoffice/openoffice in a terminal with this command:
Of course you can replace Clearlooks by any other theme that suits you more.
I created some launchers so I don't have to do that every time.
To do this, copy the original launchers anywhere into your home folder. They sould be in /usr/share/applications/.
Make em executable, by marking all the launchers in the folder where you copied them too, right click on them and go to properties - there you will find a tab for the access rights - mark as executable. Or in the terminal, change to your folder and use chmod +x:
chmod +x libreoffice*
And now you have to edit each of them (right click - properties) so it starts with the specific GTK theme. Add env GTK2_RC_FILES=/usr/share/themes/Clearlooks/gtk-2.0/gtkrc in front of the command. It should look like this:
env GTK2_RC_FILES=/usr/share/themes/Clearlooks/gtk-2.0/gtkrc libreoffice %U
env GTK2_RC_FILES=/usr/share/themes/Clearlooks/gtk-2.0/gtkrc libreoffice --writer %U
env GTK2_RC_FILES=/usr/share/themes/Clearlooks/gtk-2.0/gtkrc libreoffice --calc %U
Copy these launchers to ~/.local/share/applications/.
Now libreoffice should look fine.