For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
See Announcement
We have resolved the issue with the J! updater and this will be fixed in the next release.
I never tried mpdf.
http://fabrikar.com/forums/index.php?wiki/pdf-output/
is showing hints for dompdf, maybe it helps.
I just did a quick test.
It's not looking so bad with mpdf.
What is your Joomla template?
What do you get with Fabrik's default bootstrap?
Is it list or details view?