An interesting quote from Benjamin Britten considering many a critic has aimed similar barbs at his own operas.
But to answer your question, first about Puccini’s music in general: Puccini was at times musically lazy (and it shows up in the operas he composed music for that are rarely performed these days) and at other times his brilliance was both obvious and inspired. That he was a known playboy and your basic hedonist type has been well documented and perhaps this explains a lot of the less inspired efforts. Even so, when he was in the grove few composers of opera compare. Like with the over-performed but greatly loved La Boheme there is not one wasted note, not one frivolous, excessive passage that does not move the story along.
As to Tosca: First, being a serious writer myself, I judge opera, good or bad or average, as much by the libretto as by the music itself (which is why I find a cartoonish libretto like that for Wagner's Rings so off-putting). A few catchy tunes and some crowd pleasing arias might make for a pleasant day at the opera, but not great opera. It takes both well composed music and well-crafted dialogue to pull that off..
From Tosca: About seven minutes into the First Act:
Cavaradossi (on explaining the model that inspired his painting):
Art, with a spell of magic,
makes the two seem like one
to the beholder.
My art knows many faces,
but my heart never changes:
I have vowed my love to you,
Tosca, only you!
Sacristan’s Reply (as an aside):
It’s this one or another.
They all presume
to rival the Holy Mother.
What a stench of damnation!
Here we have two characters speaking in true character: expressing themselves as such a person would in real life. One an idealistic, educated, articulate artist: the other a cynical servant. This is inspired writing and throughout the opera it remains so. The characters stay in character, stay realistically portrayed. Coupled with the background music: music at times powerfully moving, at times subtly understated, blended near perfectly with it’s first-rate work of literature.
Tosca, in my humble opinion, was Puccini at his musical apex and easily rates as great opera.
NOTE: You might really get into Verdi's Rigoletto, which, in my opinion, has no peers when it comes an opera that explores fate, the often muddled definitions of good and evil, the consequences of misguided passions and parental protection, personal sacrifice and destiny, on so many profoundly psychological levels.