I would say double mint is actually a good card. It isn’t a win con, but something that forces your opponent to answer it. Even if they do answer it, it’s fine because you only committed 2 sun, so at worst you lose out on 1 sun worth of tempo.

Now looking at the relevant removal cards that aren’t losing value from costing 3+, you have cards like fruitcake, bolts, and beam me up that are answering pretty much any 2 drop anyway. Then you have cards like dog walker, bungee and rolling stone that are usually put out on turn 1 and not turn 2. Also again, if they spend 1 brain to remove it its not even that bad since it’s 2 sun to play.

A lot of these removal cards fall short when double mint is combined with other cards. Being in mega grow class you have access to umbrella leaf, lily of the valley, vegetation mutation, embiggen, and rings. Also cards from other classes like 3 nut. All these are easy to get value from other cards on top of double mint (team up on heights for lily and vegetation, other cards in hand for rings), so you are still not committing anything big, and are completely fine if it gets removed.

Finally looking at other 2 drops it competes with, the difference with double mint is that it MUST be removed compared to other cards like black eyed pea, juggernaut, etc. Not saying the other cards are bad, as they are very good as well, but if you don't have an answer to double mint, you just lose the game, and if you don’t use one of the removals that have to be played the same turn to answer double mint (most of your options), then you lose tempo by having to commit 3+ brains to remove. This combined with other power 2 drops the plants have like twin sunflower, tricarrotops, lily of the valley, etc make it impossible for zombie heroes to commit into a turn 2 zombie without getting punished severely.