That seems like a rather high amount. According to the statistics from Wikipedia it's more like 40-45% of all loanwords, of which French admittedly has quite a few. Crucially, the vocabulary that makes up its grammatical core is overwhelmingly of Romance origin.
what makes a language isn’t the core vocabulary but, crucially, the syntax structure. when you say ‘je suis malade’ instead of ‘suis malade’ that’s the germanic roots of french mandating the structure. latin loan words don’t make a language romantic.
French is indeed a notable exception in that regard. However, the rest of the grammar structure is still overwhelmingly of Romance origin. This and some other odd things can indeed be explained because of the ancient Franks picking up the language of the people they ruled over, but incompletely so.
French syntax structure is mostly in line with the other romance languages, with the very notable exception of not being pro-drop.
Edit: Vocabulary matters though because certain words are more likely to be replaced by loan words than others. More importantly, they allow linking with a language's close relatives (most importantly the other languages d'oïl), which might be more conservative, and with its historical forms across time. Said differently: languages without apparent similarity to other languages, no close existing relatives, and no written records to trace their development are isolates by definition.