What does it mean to say a bible translation is "evangelical?"

There's no clear meaning and the phrase should be avoided.

In one sense it would just refer to the background of the translation. Some translations are made by a team that is entirely evangelical, others are deliberately pan-denominational. There are enough highly qualified evangelical Biblical scholars that being produced only by evangelicals doesn't mean that it would be a bad translation (and conversely, including translators from many backgrounds doesn't guarantee a good translation.) But there could also be a translation produced by a small team of evangelicals who weren't really qualified to do so. Being produced by evangelicals by itself means nothing, it really matters who actually translated it. The reputable translations publish their team so you can see who worked on each book. If such a list is not published then that's a bad sign too...

In another sense it could be used to refer to the results of the translation, both positively and pejoratively. People don't like translations they perceive to be biased against them, and some people like translations that are biased in directions they prefer. Two examples of things that are commonly considered when declaring a translation is evangelical or not would be gendered or gender-neutral language, and the use of terms like "propitiation". A translation with these things might be called evangelical (even though it might not have been produced by evangelicals) and one that lacks them might be called non-evangelical (again, even though it might have been produced by evangelicals.)

The KJV and most older translations used "he" across the board. But that now appears to be male-only, so many translations switched to "they" when the pronoun referent is thought to be inclusive of both men and women. Similar changes were made for "forefathers" vs "ancestors" etc. Some evangelicals have made the misguided argument that this is a liberal influence in translation, when this is actually just good translation practice - translations should convey the meaning of the source text into the target language, and if the meaning of the Hebrew and Greek was inclusive (which it often is) then it needs to be translated into inclusive English. The NIV actually represents excellent scholarship in this regard, because they commissioned an independent report from Collins Dictionaries in order to determine what the English language actually is like now. This was a big debate amongst evangelicals in the 2000s-2010s, but thankfully I don't hear it much anymore. I'd hope that's because more people have come to understand that good translation involves responding to changes in the target language, but it could just be that the two camps aren't talking to each other anymore.

Another example of something that is called evangelical or non-evangelical is how words like ἱλαστήριον (hilastērion) get translated. Most evangelicals believe that Jesus performed a propitiation in his death, while many non-evangelicals reject that concept of the atonement. So it's simple right, the translations that use the word "propitiation" are evangelical, and those that don't aren't? Well of course not. You can compare translations of Romans 3:25 here, and you'll see that many translations produced by and for evangelicals don't have the word propitiation, including the NIV, the NLT, the CSB, and others. Remember that translation is about effectively conveying the meaning of the source text into the target language. Some evangelical translators have concluded that "propitiation" is not the best translation for ἱλαστήριον, maybe because they think the English word has a meaning that doesn't really match the Greek, or maybe because they think the English word is too technical for use in a translation intended for average English readers (and of course including many ESL readers too!) The doctrine of atonement by propitiation does not depend on how one word in a few verses is translated but upon numerous other places in which the wrath of God is mentioned.

We should not be choosing translations based on surface level features which align with our theology, but on how rigorous the scholarship behind them is, and how effectively they communicate in the language of the average English speaker. If we all did that then translation labels like "evangelical" wouldn't matter.