Alright, first up is Wormadam.
A three-for-the-price-of-one Pokemon, Wormadam was one of the least used fully evolved pokemon in its debut generation, Diamond/Pearl/Platinum, and still is today.
Wormadam comes in three forms, each with their own typing.
Wormadam-Plant:
A sorry excuse for a pokemon. Bug/Grass type. Poor stats overall, like many of the overshadowed weak bug type Pokemon out there, it has nothing to set it apart from its competitors. Parasect is slightly better, with a wider movepool and access to the always good Spore, and more balanced stats. For reference:
Wormadam-P:
60/59/85/79/105/36, for a total of 424. Those lopsided stats are quite frankly awful. 60 HP is fragile. 59 attack is basically useless. 85 defense is not terrible, but it is nothing special. 79 special attack could be somewhat usable, but you're not going to be hitting hard with this little thing. 105 Special Defense is good, but there are other defensive pokemon out there with better movepools, better stats, and better typing. Its speed is basically non existent, so I guess you could use Trick Room and laugh as it still can't do much?
Basically the only thing Wormadam-P has going for it is spamming Choice Specs boosted Leaf Storm. That's it. I mean, I guess it can set up Stealth Rock, but I doubt it'd survive long enough to even do so.
You know it's bad when it's outclassed by a middle evolution; that being Roselia. Smogon's suggested moveset is Signal Beam/Leaf Storm/Giga Drain/Stealth Rock, with Choice Specs as the item of choice, but they plead with you to choose basically any other bug or grass type, as even in the bottom of bottom tiers, Wormadam-P is totally outclassed.
If you have gotten this far and still want to use Wormadam— please do not; use Victreebel, or Tangela, or Roselia instead.
Moving on, we have Wormadam-Sandy.
Wormadam-S is a Bug/Ground type, an uncommon type. It is still trash, but not like its other form Wormadam-T. Wormadam-S is slightly notable because it looks like a slice of pizza kind of.
Its base stats are the same as the other Wormadam-forms, but shuffled around.
Wormadam-
Sandy Pizza form has the following base stats:
60/79/105/59/85/36.
This, much like Plant, is kind of weird and lopsided. You can clearly tell Game Freak wanted it to be defensive, but it got slapped with a poor defensive typing in Bug and not many tools to use.
Unlike Plant, Pizza can survive in a sandstorm without taking scratch damage, and uh, not much else. Unless Plant has its second ability, Overcoat, but that really isn't going to do much.
I guess it's like a weaker Golem that isn't rock type and instead is Pizza type. Its recommended moveset is "Margherita", with Stealth Rock, Earthquake, Rock Blast, and Sucker Punch. "Golem can do the exact same moveset but not suck with it." Pizza form doesn't even taste like pizza, it's just the bitter gritty taste of defeat.
Sucker Punch lets you lightly poke an enemy if they're about to attack Pizzadam before the little bagworm inevitably faints, Earthquake is old reliable, but it's a pretty weak earthquake, like the ones we get here in Australia, not the ones that stronger pokemon can fire off. Stealth Rock is Stealth Rock, and uh... Rock Blast is the only thing Pizzadam has to deal with pesky flying types that would eat it right up, getting a mouth full of sand in the process. Of course, being so slow, it's unlikely Pizzadam would survive long enough to throw rocks at the birds anyway. I guess Pizzadam can learn Fissure, and it's the lightest pokemon that can do so, but uh, if you're seriously considering putting a 30 accuracy move just so Pizzadam can get lucky and actually do something with its life, please reconsider.
Last up we have Wormadam-Trash.
Don't let the name fool you, Wormadam-T isn't literal trash, it's only figurative trash.
Wormadam-Trash is actually pretty cool, I guess. It's like a bug that looks like fairy floss, and it's got an actually good typing in Bug/Steel, unlike Pizza and Plant. It still has the glaring flaws shared with Pizza and Plant, but they're mitigated somewhat by its typing. It's even slightly bulkier than the other two, making it actually kind of defensive instead of "we thought it could be defensive but then laughed at it".
Wormadam-T's base stats:
60/59/95/69/95/36
This is not terrible, I guess. It's much bulkier than its siblings, and probably won't die in one hit maybe hopefully. That's about where the good stuff ends. It has next to no offensive presence. It has no reliable method to heal. Other steel and mixed-steel type pokemon outclass it horribly, most notably Probopass. It can't really do anything against pokemon that like to boost their stats. Its recommended moveset is:
Stealth Rock/Toxic/Protect/Iron Head
Stealth Rock because it's always good, and Trash might actually survive long enough to get one out. Toxic is useful for defensive pokemon, as always, because it puts the enemy on a short timer before they faint, and it's Trash's best way of dealing with anything that might threaten it, like every pokemon in existence. Protect is to stall and let Toxic rack up damage, and also because Trash doesn't have any other good moves to put there. Iron Head is so it's not totally stopped by Taunt, but with 59 attack it's not going to be doing much but lightly booping the opponent with it.
I guess Overcoat is kind of useful against enemies that would use stuff like Sleep Powder, like Venusaur, but considering nearly everything can deal with Trash without having to put it to sleep, Overcoat is basically wasted on it.
Thus: In general, Wormadam is trash, figuratively speaking, regardless of its form. Plant can click on Leaf Storm and tickle the opponent slightly better than its siblings. Pizza/Sandy can do a Golem style set, but badly. Trash can while away its hours (realistically seconds) of life with Toxic and Protect.
I guess they're kind of a cool idea, but being that they were part of the classical "early route bug trash" archetype, Wormadam and its male counterpart, Mothim, (yes, very funny) they're not going to do much.
Oh right, before I forget, Wormadam and Mothim both have access to Quiver Dance, but realistically, do you see them surviving long enough to get more than a single boost off?