Claremont got super bored of Asian Psylockes and had a big storyline planned for Xtreme X-Men where Vargas (evil rich swordsman who claims to be the pinnacle of human evolution who wants Destiny's diaries because a psychic told him an X-Man, was destined to kill him kills Betsy and steals her corpse/turns it into a bronzed statue in his evil lair. Which was all a long plan set up to have Betsy resurrect and when her bronzed statue shell broke upon her resurrection, she'd be British/a Kate Bush lookalike again.
But around the time he set up the arc, Morrison got approval to fake kill Magneto and to hammer home the idea that Magneto was REALLY fucking dead forever and not the new masked Chinaman named Xorn that just happened to join the X-Men, Quesada passed a new decree stating "dead is dead" and that going forward ANY dead character could not be resurrected PERIOD.
Which came AFTER Claremont offed Psylockes to set up the resurrection plotline.
Naturally Claremont was pissed the fuck off, not only because Quesada effective fucked him over with no lube since this meant he killed Betsy for absolutely NO reason and now forbidden to backtrack on it. Which was a BIG thing, as Vargas was supposed to be the big bad of Xtreme X-Men and Betsy's death/rebirth a major storyline relating to the Destiny's diaries and the fact that Betsy was supposed to kill Vargas and that by offing her, he basically sealed his own fate in terms of Betsy killing him in return.
Adding to this, you had shit like Bendis in New Avengers and Secret Wars 4 having dead villains show up alive as background characters and Ed Brubaker demanding and being allowed to bring back Bucky. And of course, Morrison bringing back Magneto and bragging about how it was always planned for Magneto's death to be temporary and Whedon being ordered, as a condition of writing X-Men, being told to bring Colossus back to life. Which only made Claremont fucking rage every MORESO at how he got fucked over and fucked over HARD.
Finally, after Morrison left and Whedon brought back Colossus, Quesada THEN finally decreed that Claremont could "bring Betsy back". But by this point, Claremont had been forced to heavily rewrite his plans for Vargas and end it with Rogue being the one destined to kill him not Betsy and then deal with the fact that Vargas was so unpopular BECAUSE he killed Betsy, that editorial had ordered him to wrap up the Destiny's diary storyline and start aping Morrison's shit in Xtreme X-Men.
Which led to Claremont doing an absurdly lazy return storyline where Betsy basically magically teleports into the X-Mansion alive during his fourth X-Men (and his final Uncanny run) and well and still in her asian body (since Claremont wanted to give a legit reason for her getting her old body back and was denied this), with no explaination until the very fucking end of his run where he credits her return to Jamie Braddock and then teleported her off to be involved in Exiles and New Excalibur shit no one read and gave a fuck about, until Matt Fraction brought her back to the main X-Books during his run.
Also, Byrne's worse than Claremont in that Claremont's more passive aggressive than actively malicious, as Byrne (like Bendis) breaks characters so fucking hard and with such explicit malice to render them radioactive, simply because he can do it and use his name and clout to make it impossible to undo what he did.
Claremont's Xtreme X-Men run was filled with take thats and fuck yous to Morrison and his X-Men run that ran at the same time and was infamous in that Morrison denied Chris usage of Beast and even tried to take Gambit/Rogue away from Claremont so he could kill Rogue and replace her with an overly cliched loli goth who's design ended up being recycled for Teenage Negaton Warhead. But nothing like the shit Byrne has done over the years to characters he dislikes or feels the need to "fix".
I didn't really feel it because it came out of fucking nowhere. She had been around for like 20 years at that point, almost a decade of having sorta joined the X-Men and talked and behaved normally then Morrison has her bust out the Britishisms because reasons and start behaving like a sleazy whore. I don't really care all that much about Emma Frost to be annoyed about it but if you were a major fan of her character, I can absolutely see why you'd be mega-pissed at what Morrison did to her character.
The hate for Morrison making Emma talk like a Brit comes from the fact that it flies in the face of pretty much EVERYONE ELSE writing Emma as an old money Bostonian type New Englander and the fact that Claremont and Lobdell basically wrote Emma as sounding like Kathleen Turner, who was the post-Byrne visual inspiration for the character under later artists, along with Faye Dunaway (who Byrne used as Emma's visual basis). Combined with the fact that Claremont and Lobdell explicitly wrote Emma as being in her late 40s (making her around the age of Xavier and Banshee, who is canon older than the rest of the AN-AD X-Men save Wolverine) whereas Morrison ALSO deaged Emma to be in her early 20s so her relationship with Scott would be as "equals" age-wise instead of Scott fucking a woman that was twice his age.
TL;DR Morrison basically ignored a LOT of canon with Emma to the point of turning her into a borderline INO version of herself.