It depends on exactly what you mean by ‘important’ (important to who, and for what purpose)…
As @mikeloc points out, a TLD shouldn’t affect SEO, but there are some other things to consider.
.com is the one that everyone knows, and will likely be the easiest to remember (and if people don’t actually know your TLD, they’ll probably use .com by default, or by mistake), so .com is still great if you can get it (even if you don’t actually use it as your main domain).
Obviously, if your app/website is aimed solely at a particular country then using the country’s TDL is probably better than .com.
Also, there are some TLDs that have associations with particular industries or app types, that may be better, and more descriptive of your service, than using .com, and some that just sound good with your domain name (some websites even use the TLD as part of their name).
Personally, I like .co (when .com is not available), but one thing to consider there is many people will either think it’s a typo (and type .com instead), or they will just add the ‘m’ out of habit.
So, ultimately, it’s important in the sense that it will become part of your brand and identity - so you need to be happy with whatever one you pick…
But it’s not really important at all in terms of how your app will rank or perform (I’d still avoid obviously spammy TLDs, or ones with bad reputations, so do your research) but other than that, just pick one that you’re happy with and go with it.