Whitney - Flaaffy sucks against Miltank, even with Thunder
Thunder Wave is a huge contribution against Miltank because it can stop Rollout from building up and it slows her down so Stomp flinching isn't an issue. Even if Flaaffy can't beat it outright, it can cripple it enough for someone else to clean up.
Morty - mediocre matchup, best you can really do is paralyse his Gengar for someone else to KO but Flaaffy is too weak to sweep here
Barring I dunno, Kadabra, what mons outright sweep Morty for free? It isn't a great matchup though, I concede that.
Chuck - struggles to KO Poliwrath without Thunder and gets destroyed by both Surf and Dynamicpunch, even if it's evolved by this point
"Even if it's evolved"? Ampharos struggling to beat Poliwrath? Sorry, I just don't see it.
Jasmine - not a great matchup unless you have Fire Punch, in which case it's fairly even
Fire Punch is enough to deal with the TBolt Magnemites, that's actually a lot. Sure, Steelix might be too much, but not soloing a leader doesn't mean it's bad, especially if your plan to deal with Steelix is a Water-Type like Feraligatr.
I'm not advocating for Mareep to shoot to S-Tier, but it's definitely not a bad mon. It actually deals with a lot of potentially problematic mons and it's just good at route clearing because of how good Electric+Fire is offensively.