Is there any point to stack Tougher Times past 7?

The teddy bear item adds 15% block chance per stack. This means that at 7 stacks, you'll hit 100+% block chance. Unlike the Lens-Maker's Glasses, the additional sentence at the end has me wondering what it means.

It says "Unaffected by Luck". But if you block all damage 100% of the time, wouldn't it render the rest of the game pointless since you can no longer take damage?

Yes, but you'll need around ~6k to get close to 100%.

Doing some research, it turns out that the 1st teddy bear you get adds a flat 15% block chance, but each additional bear after is not actually additive, but multiplicative.

The datamined equation is: (1 - 1 / (0.15 * n + 1)), where n is the number of Tougher Times you have. The graph below from the datamined source showcases that in effect:

Basically, you'll need to stack ludicrous amounts to come close to 100% chance, but no matter the amount of teddy bears, 100% block chance can't actually be achieved.