Over muddling is a problem. The idea of muddling is usually to express flavors. You can go crazy with berries – you can’t over do that. However, citrus and most herbs, including mint, have bitter parts. So while your expressing those flavorful oils from the lime, you’re also releasing the bitterness from the pithy sections. So, take it a little easy.
I have found that a lot of flavors can develop just from shaking with ice cubes, (not crushed ice). Especially mint. Just smack the mint in your hands and let the ice do the rest. Makes for a great mojito.