Revamping Grimer,as the one on Page 1 is really...um...misleading.
Grimer-Low
Availability: He's found in Pokemon Mansion, though he's easier to find in Blue and Yellow. It should be noted that you can also catch fully evolved Muk there, too.
Stats: 105 HP and 105 Attack is very nice, but base 65 Special is terrible. 65 Speed is usually enough to outspeed most things in-game, so it's not entirely terrible in that respect.
Movepool: If he doesn't have Sludge yet, he'll have it in a couple levels. Aside from that, he gets Thunder, which no one really wants anyway, and does decent damage off his Base 65 SpAtk stat, I guess. He also gets Acid Armor, which makes him a better defensive Pokemon than Weezing against stuff like Fighting types and can even contend with Ground types barring crits. He gets Body Slam as well, and can make good use of it, but you've probably already given that TM to someone else...
Power: Grimer's terrible, but Muk's alright thanks to that good Attack stat. Poor speed blows, and a movepool filled with special attacks isn't helping it. It literally has like three physical attacks to choose from; Sludge, Body Slam, and Hyper Beam. If it had a better Special stat, it'd probably be able to use some of those TMs more efficiently...on the plus side, Mega Drain hits Rock/Ground types hard
Typing: You've passed the point where there were a bunch of Grass, Bug and Fighting types, so Poison's kind of a meh typing. It also kinda sucks that Psychic types are in God tier,
Matchups:
Blaine- Growlithe, Ponyta and Rapidash can be taken out by Muk because they lack any decent Fire moves (other than Fire Spin, I guess...) Arcanine packs Fire Blast, so don't even try against him. In Yellow, the only thing you can take out is Rapidash.
Giovanni- Nidoqueen and Nidoking pack terrible moves in Red and Blue (they don't have a single Ground type move), so you're safe against them. You also beat Rhyhorn and Rhydon with Mega Drain because of their atrocious Special Stat and your high defenses. Be careful though, Rhydon has Fissure. In Yellow, the only thing you can kill is Persian.
Lorelei- Can't really hurt her Pokemon at all thanks to Muk's low Special and her Pokemon's high defensive stats.
Bruno- Muk beats all three Fighting types and can take out his Onix duo with Mega Drain.
Agatha- Everyone on her team resists Sludge and are immune to Normal type moves. They also pack high Special stats, so I'm not seeing a Muk victory here. Not to mention one of the Gengars packs Psychic.
Lance- Muk can beat the Dragonairs and Dragonite because the AI is an idiot and always spams Agility when it's out. Gyarados and Aerodacytl hit too hard, though.
Rival- His Exeggutor is garbage and only has three moves (none of which are any good), so Muk beats that one. Pidgeot's also garbage and Arcanine has a garbage movepool, though Ember still hurts a bit. Again, Rhydon dies to any Special Attack, so Muk's Mega Drain kills it. In Yellow the only thing you can kill is Exeggutor.
Additional Comments: Muk's not bad, he's just disadvantaged. He only has one good moveset; Sludge, Mega Drain, Body Slam, Toxic. He's also disadvantaged in having a low Special in a game where Special is everything. Add to the fact that he has the worst type in the game...yeah, not seeing the appeal here. He's a decent check to Lance, and does better against Giovanni than one would expect. His biggest flaw is that low Special Stat, meaning that Mega Drain really is a move he wants so he can at least butcher the random Onix or Rhydon.
Frankly, I /could/ see it in Mid, but a very low Mid.