I've seen people struggle to pronounce a word when I explicitly tell them what tones it contains, but then pronounce it perfectly when I ask them to just imitate me.
But I disagree about accents. One of the major flaws in most foreign language education, in my opinion, is that pronunciation is not emphasized heavily enough at the beginning. Being able to pronounce the basic sounds correctly has a huge impact on how native speakers perceive your language skills, even if you're not very advanced in the language.
That's true, but it counsels against trying to develop better pronunciation early.
If you sound like a native despite having just started to learn the language, people will naturally conclude that you are mentally retarded.
(1) It doesn't get any more difficult to fix your accent. But most people won't, because there's virtually no benefit to doing it.
This is related to
(2) Once you learn to speak a language, you're not at any risk of people thinking that you can't speak it, even if you speak with a strong accent.