One thing to take into account is that singer Mariah Carey actually has more #1 hits than Elvis. I also think that Mariah's accomplishment is more impressive, as she came out as a singer, songwriter, composer, and producer; while Elvis was merely a heavily marketed pop performer - similar to Katy Perry and Britney Spears. She just came out there, and completely wowed the crowds with her impressive perfect pitch singing voice with a range of over 5 octaves. She did it without the wacky suits and dancing too (well, at least until years later in her career); the dancing and the weird get-ups were among the key selling points of Elvis's performances.
Lady Gaga at least is a writer/composer; and I think a lot of the negativity towards her is because of sexism; whenever a female singer gets popular, you see LOTS of detractors trying to bring her down, because they find her threatening. It happens every single time. Mariah Carey is proof that a singer can have more #1 hits than Elvis - even in light of that sexism among music "fans". Lady Gaga doesn't have the same gifts that Mariah had, but then again, neither did Elvis.