Underrated? No. Underdiscussed or undertaught, yes. His overt racism makes open discussion difficult. But anyone who reads horror seriously very much appreciates his work.
It is possibly a religious thing. The puritan/protestant influence on american culture tends towards down-to-earth fiction that avoids discussion of fantastic situations that rub up against religion. Lovecraft's basic mythology is that there are things older than the earth. That conflicts with creationism and so would be a shunned in 18/19th century america. Such things certainly would not be found in school libraries.
The old people here might remember the anti-magic push in the 90s, the satanic panics and efforts to stop Halloween. Midi-chlorians were part of this, removing magic in place of scientific explanations. Then harry potter swept the movement away.
AIUI France took sci-fi somewhat more seriously than the anglosphere did from earlier on (maybe due to Verne?); in English-speaking countries it was really seen as pure genre fiction with negligible literary merit until quite recently.
