Tifa absolutely and categorically is a truly important character in Final Fantasy VII and literally always has been and always will be.
Counter to Aerith and the theme of "how do you overcome loss?", she is the heart of the secondary theme of FFVII.
Which is the theme "what does it mean to be a hero?". How you achieve your hopes and dreams, by loving and protecting others.

Tifa is the love of Cloud's life, and as his muse and inspiration since his childhood, as well as a source of emotional turmoil from shouldering the burden of being shunned by the town.
Tifa quite literally physically exists as Cloud's hopes and dreams to become a SOLDIER made manifest.
She is the only true motivation that he has ever had, to break out of his insecure shell and become the hero he was destined to be.

And despite all of that being clearly laid out in crystal clear English in Final Fantasy VII across the flashbacks, 28 years ago, anons will indeed shout down this very concise point I just made.
"Contrivances". Like any single character in a fictional narrative isn't contrived, whittling out all circumstance.

Aerith is totally contrived. Cloud falling through her church was 1000% more contrived than Tifa literally stumbling across him at the heavily populated train station.
Yuffie is also contrived in her entire existence, period. She's a comedy character.
Barret is contrived. Red XIII is contrived. Hojo is contrived. JENOVA is contrived. Sephiroth is contrived. Shinra Electric Power Company is totally contrived.
The Lifestream is contrived.
This word is perhaps one of the most useless things you can say about any fictional character or idea.
It basically says "I'm not capable of discussing anything about what this idea is supposed to represent and communicate to the player".

tldr; begging you to gain a crumb of media literacy