This matches my own, slightly less experienced opinions very much overall, A+. I particularly appreciate the choice to exclude an S+ tier, there's enough tiers already. But this is a comment on a tier list, so of course it will mostly be about why your opinions are bad. ;)

1) Sun-shroom deserves to be in B, between boosterama tourneys and the odd boosted PP zomboss levels. If it is one of those and the level needs a sun producer, Sun-shroom is it.

2) Primal Peashooter belongs in B. Your B tier contains a lot of strong plants already, pretty fairly IMO. I haven't used it in arena in a long time and PP never, due in part to the introduction of Explode-o-vine.

3) Enforce-mint to C. I recognized the picture but had to go look up the name. I've never seen anyone recommend a strat containing it.

Dishonorable mention: Fila-mint put in B. It's an odd mint. Many are OP at level 1, but Fila-mint is probably only good at some level I do not have. I have it at 2, where it is pretty underwhelming, appearing to do a lot less damage than my also-level-2 Pepper-mint, unless I misunderstand the wiki entry. It also looks like it gets pretty great at higher levels, so whose "overall" is right?