I had a similar problem, and even though 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), 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 with 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 should be in /usr/share/applications/.
Make your copied launcher(s) executable by right clicking on it in a file manager and going to "properties". From there look in the "access rights" tab and click "mark as executable."
Or in the terminal, change directory to your folder and use
chmod +x libreoffice*
And now you have to edit each of them (right click - properties) to add the specific GTK theme to the beginning. 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.