I think when you are weak in a particular area there are a few things you can do. First you can work to improve it. That is always worth exploring and I think there is nothing in this world quite like taking a junk box and turning it into a corvette. So if you are bad at impressions, but have a strong desire to improve, by all means work on it. Another solution is to ignore you weaknesses and play to your strengths. Nothing is wrong with that approach either, I use it all the time. My favorite approach, when it makes sense to do, is to turn a weakness into strength.
I am very bad at impressions. Not too good at accents either. This used to frustrate me a bit, because I liked the idea of doing interesting voices at the table. Now I've learned to accept it and turn it into a strength because when you can't do impressions, no one knows you are doing an impression.
If I try to act like Robert De Niro or Jerry Seinfeld, my players will have no idea who I am supposed to be. This is annoying if you want your players to believe you're Robert De Niro, but since I've never run a campaign where meeting Robert De Niro was a remote possibility it isn't a concern (the closest we came was a Crime Network game where one PC tried to break into the music business). If you just want your players to believe you're playing distinct and interesting NPCs, then a bad impression can go a long way.
I've packed my campaigns with guest appearances from the likes of Peter Ustinov, Gilda Radner, Jim Carey, Carol Kane, Vincent Price, Katharine Hepburn, Charles Laughton, Sigourney Weaver, Sean Connery, Don Knotts, and many more. In none of these cases was my portrayal of these people at all convincing. In fact my Sigourney Weaver is probably not all that different from my Peter Ustinov, but it is different enough that I can see the characters I use them for as separate, which helps me get a handle on them each time they come up. This also helps my players know whose voice I am speaking in, even if I have two NPCs engaged in a discussion with them.
Generally I limit this mainly to minor NPCs I need to flesh out on the fly. Impersonating a celebrity badly is a great way to quickly put together a believable NPC. For major NPCs, I think you need more of a solid foundation than just the voice of a celebrity. But for a barkeep or bandit, it can really help to emulate the mannerisms of a famous actor, comedian, politician or even a musician.
So next time you fail at a celebrity voice impression or trying to sound like a lizard man, turn it into a strength. Allow your lack of skill to cloak the source of your inspiration.