This is not true. Does AV Tangrowth suck on stall? Definitely. Its only means of team support for stall teams is Knock Off, making it a magnet for dangerous offensive MEvos that are very difficult to be contained, such as Mega Pinsir, Mega Char Y / X, and Mega Lucario. Now try using it on a bulky offense team and you will see the difference. AV Tangrowth checks a ton of Pokemon, providing a lot of error room for your team, and can form a great defensive core with just one more Pokemon (Rotom-W is a good one), leaving your team free to focus on offense, hazards, momentum, etc. Also, being an incredible pivot that can switch in and out at will is marvelous, as it can provide a lot of free switches to Pokemon that can actually make a difference with one free turn, unlike stall mons.
tl;dr AV Tangrowth sucks on stall but is great on bulky offense.
As for good answers to Mega Pinsir on stall, don't forget Zapdos people, which also has Defog.