One number that suggests otherwise is box office data. Domestically, using rounded numbers, Alien's $80 million in 1979 would inflate to $285 million in today's dollars. Aliens' $85 million in 1986 would inflate to $200 million in 2020. In raw numbers, Alien grossed $203 million WW versus Aliens' $183 million. This would suggest that Alien is the more popular of the 2, at least in regards to how many people saw each film in theaters.
In regards to how well liked each film is, that's murkier. Obviously Aliens wins the vote here, but it's not a head to head matchup. Alien has a higher RT score (98% vs 97%) and also leads on IMDb (8.4 vs 8.3), but those margins are razor thin.
I do feel the Alien franchise is comparable to the Terminator franchise in that non-fans generally regard them as 2 great films & a bunch of sequels. The difference is financially, T2 drastically outperformed The Terminator, whereas Aliens slightly underperformed Alien.