You don't need to use link shorteners on Mastodon!
All links on Mastodon count as 23 characters towards your post limit, no matter how long the links actually are.
Link shortener services like bit.ly, t.co etc track users who click on their links. This is really bad for privacy. If you use link shorteners on Mastodon, people may assume you are just doing it for tracking purposes.
More info at https://fedi.tips/you-dont-need-link-shorteners-on-mastodon

fedi.tips
You don’t need link shorteners on Mastodon | Fedi.Tips – An Unofficial Guide to Mastodon and the FediverseAn unofficial guide to using Mastodon and the Fediverse
p.s. Twitter forces all links to go through its own t.co link shortener. Most people probably don't realise this, because the t.co links just look like normal links.
You can see this on Firefox on a computer. Hovering the mouse over a link shows its true URL in the bottom left corner.
On Twitter, all links are actually t.co addresses, so Twitter will be able to track people clicking on them.
On Mastodon, links are exactly what they appear to be.
See the screenshots for a comparison.