First off, I'd be willing to be Brit isn't much of a singer to begin with, *but*...
In all likelihood, she isn't getting a monitored feed from that mic...no in-ear monitoring, because her singing isn't going to go out over the mains, and she doesn't need monitoring for when she is just speaking to the audience.
All this to say that, even though she isn't much of a singer, if she's dancing and standing amongst all the cacophony of crowd and PA system and such, she can't hear what she's singing - even for lip sync purposes - so she has no way to even remotely stay on pitch. Throw in dancing on top of that, and yeah...she's gonna sound like shit.
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"To those of you who are wearing ties, I think my dad would appreciate it if you took them off." - Robert Moog
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