Garbine though so far has been wildly inconsistent, and has only won 4 titles in her career, with 2 of them now being majors.
Yeah, she certainly isn't the first player to have all the tools necessary to win majors but struggle with consistency in an offense-first style of play. That said, here's the list of ladies in the last 20 years with multiple major titles before the age of 24:
Martina Hingis
Lindsay Davenport
Venus Williams
Serena Williams
Justine Henin
Maria Sharapova
(Capriati's peak was 25-26, Mauresmo 27, Clijsters 26-28, Azarenka 23-24, Li Na 28-32!, Kvitova 24-?, Kerber was 28 last year)
Everyone in that list is going into the Tennis HOF; Hingis, Davenport and Henin are already in, the others are merely waiting for retirement. Muguruza is 23, and with her results, her floor appears to be the injury-riddled career of Azarenka's mid-to-late 20s. With health and coaching, she could quite possibly be in the Sharapova-Venus-Henin tier of "borderline top-10 all-time".
Of course, everyone wants to compare her to Serena. Entering the spring of 2002, at age 20 (!), Serena had the '99 USO to her name, plus had made the QFs of every slam in 2001 (and was runner-up in the USO). She then won 5 of the next 6 titles including the next 4 straight. That's a completely unfair standard. As they turned 24, Graf had 12 major titles (!), Evert 8, Serena 6, Navratilova 2. I think we can agree Muguruza's trajectory doesn't put her on the Mt Rushmore of women's tennis, but it should surprise nobody if she ends up an outer-circle HOFer.