Lopunny is underwhelming because it's an anthropomorphic rabbit.
It's Mega certainly has the aura of someone meant for combat... but being made into a part fighting time can certainly be part of that.
I've just not heard a lot of praise over Lopunny as a Pokemon.
Heracross is just one of the bigger names of Gen II, being part of Ash's team probably helped with that.
With Blastoise?
C'mon now.
Most if not all starters are some of the most popular Pokemon of their region.
The Kanto starters will ALWAYS be some of the most popular Pokemon.
And, especially in the west, Venusaur got tossed a bit to the side.
Among fans, it's been Blastoise and Charizard as the big two titans for the starters, one of few rivalries of the fanbase.
Why is it popular, besides merit of being a Kanto starter?
It's a big *** reptile with giant aqua cannons. AS KIDS THIS WAS THE COOLEST CRAP AND WE STILL LOVE IT.
Google image search Blastoise, your suggestions are "Mega evolution" "card" "vensaur charizard" then "cool" as the big 4.
The likes of Lopunny... "R34" "Mega evolution" "lucario" and "gijinka."
Blastoise is a nostalgic staple of a "rule of cool."
Lopunny has a smaller fanbase known for different reasons.
This is like comparing Wolf to Krystal, almost.