There does not seem to be any evidence that the Hoff is more popular in Germany than other countries. While living in Berlin for a few years I certainly never saw any exposure to him than what might be considered average.
The idea may have come from the fact that he was popular at the tail-end of the 80's more so than other countries and had hit , Looking for Freedom, that was at number 1 for about six months.
It could also be that he performed this song during celebrations for the wall coming down, when his popularity had already peaked in most western countries.
Not to mention Germans being enthusiastic fans of Hasselhoff is a good example of a self-propagating meme, which was no doubt helped along by Norm McDonald incorporating the trope into his skits on SNL.
It's interesting to observe that doing a Google date range search for "hasselhoff germans love" and similar phrases for 1980-01-01 to 1993-01-01 returns nothing, while a similar search for 1993-01-01 to 2000-01-01 returns many results. This is of note because Norm McDonald started at SNL in 1993, with "Germans love David Hasselhoff" being incorporated into his skits since at least 1994.
This chart on the German Wikipedia shows the chart positions for Hasselhoff's different albulms. Chart positions in Germany generally trail behind Austria and seem to average out to be similar to Switzerland. Chart positions are not the only indicator of an artists popularity ion a country, but they help in giving an indication.
It doesn't look like any study has been done on this topic. Given the evidence we do have, there is nothing to indicate The Hoff is more popular in Germany than other countries.