These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Typically portraying her as a yandere who wrote the letter herself.
Interestingly, if you watch the series Kat's feelings are one-sided and she understands that Tommy is still pining, so despite her own wants she tries time and again to find Tommy a new love interest who ISN'T her. Eventually he realizes on his own what he's got in Kat. Word of God even says this is intentional to keep Kat from looking like some heartless boy stealer.
What's really funny about it is that Nakia Burrise has a great singing voice. Jason David Frank... does not (although, some have said that Frank's singing voice is tolerable; he just thought it would be funnier for Tommy to be hideously off-key, which it was.)
Game Breaker: The Zeo Ultrazord. Exactly ONE monster ever managed to put up a fight against it.
The Super Zeo Megazord also qualifies, as it was incredibly rare that a monster held its own against this mammoth megazord, which stood almost twice as tall as a giant monster. Even the series Big Bad King Mondo fell to it. He got better afterwards, but still.
Unfortunate Implications: The crew decided to reveal that Tommy had Native American heritage at the same time he became the Red Ranger while emphasizing it in a way that basically consists of "brothers, an arrowhead, and Eldritch Abominations", with All Natives Are Lakota thrown in. Granted, Lakota myths do feature brothers taking on an Eldritch Abomination, but still...
Behind the scenes it was just an excuse to get Eric Frank a guest appearance on the show.
Villain Decay: While Zedd's marriage to Rita is argued if it is truly an example (Post "decay" Zedd having done more damage to the Rangers), Zedd and Rita being forced to travel around in a camper is a pretty solid example. Thankfully it doesn't last and by the end of the season they're no longer afraid of the Machine Empire at all and Rita manages to create a monster that beat even the Ranger's ultrazord!
Played straight with the Machine Empire. They are initially built up a a threat even greater than Rita and Zedd when they chase them off, but despite showing they have an army and also statements of conquering whole galaxies, we never see them do much besides Monster of the Week plans, which they had less success with than Zedd and Rita. Rita even lampshades this in the finale when she says their not afraid of them anymore with how that be then they've become a joke. This change happens even earlier in the series than it usually does, following their first two monsters (including one that nearly killed the Rangers), they went from a deadly force that was intent on Earth's conquest to filling Rita and Zedd's shoes as Evil Is Petty creators of the Monster of the Aesop, focusing more on disrupting the everyday lives of the rangers than destroying them.
What an Idiot: After firing the ultrazord blast against Impursinator, the monster swells up so much that it nearly explodes from absorbing the energy, barely managing to reflect it. It was later revealed that they could fire consecutive finishing blasts in their battle against Prince Gasket and Archerina so why didn't they hit it with two in a row after it saw that one almost killed it? Of course the answer is Sentai limitations, but the same could be said about Oh Ranger.