Is there any evidence that 90% of characters are phono-semantic compounds?
It is actually not possible to provide an unambiguous statistic like this without either being ignorant of the complexity of character development, or pushing some kind of agenda. Victor Mair and John DeFrancis have been spouting this kind of idea for a long time, but I suspect that they've either (1) focused too much on lexicography and not nearly enough on paleography to make such a conclusion, or (2) wanting to blow up the significance of spoken language and diminish the role of written language in the history of a language's development.

In real life, characters are not cleanly divided into the liushu categories, and the classification is quite deficient to fully describe many characters. 90% seems like an overestimation, but one can actually arrive at this number by interpreting any character with any amount of history of involvement with phonetic components, no matter how slight, as a "phono-semantic compound". However, if someone follows this interpretation, it is misleading to use vocabulary from liushu (like phono-semantic compound) to describe these characters.

There are example of characters (e.g. 「源」, 「採」) being both compound ideograms and phono-semantic compounds, as categorised as a type of the latter, but there is rarely an obvious mention that it is a type of the former as well. There are also non-obvious cases; for example the characters「喪」and「鬱」, under an opaque interpretation, can both be described as "phono-semantic compounds" (and indeed have been in traditional sources):


「喪」had a history of development with phonetic components「桑」and「亡/亾」(see an answer I wrote at Japanese StackExchange, about halfway down), but note that「亡」also provides a semantic indicator;
「鬱」was originally a graphically simple compound ideogram, but became complexified over the years (see the Xiaoxuetang entry for a rough overview of the evolution).


Nobody recognises the phonetic components in these anymore, as they are either very rare components or have been graphically corrupted too much over the years, but they're categorised as "phono-semantic compounds" anyway. I personally find such a liberal interpretation as bordering on meaningless, as it is equally possible to provide a liberal interpretation of "pictogram", "ideogram", or "compound ideogram" to misleadingly inflate a statistic.