If you're going to use vague terms like "pretty popular", then there's no way to actually argue against it. The fact remains however, the the Nazi's, even when broken down by region, only managed 50% of the popular vote in any area one time, and that fell by a solid 10% in the next election (which is coincidentally the last free one) whereas another party managed to do that on multiple occasions (The Central Party in 1919, 1920, 1924, and 1928)
As for Reagan, you have to look at percentages. He's the only President in my lifetime (I'm right on the gen-X/millenial border) who has beaten his opponent by 10% or more in the popular vote.
If you want to make that comparison, you have to look at what the other German parties were doing. The Social Democrats, the leading party from 1919 until 1932), was always in the 20% range with a low of 20.5% in 1924 and a high of 29.8% in 1928 with most of the time spent in the 24% range. The next two major ones (the Centre and Communists) both also stayed pretty steady at 11-13% and 10-13% respectively. Contrast that with the Democrats around Reagan. Carter had 50% in 1976 but fell to 41% in 1980 with Mondale having a similar performance.