When viewed in context, that's not as good as one thinks. Trump's approval rating, averaged across multiple pollsters, has hovered in the low 40s for the past eight months. Reagan, and Clinton were at or near career lows at this point. It's not so much that Trump caught up with them, as theirs had dropped to Trump levels at this particular point in their terms. Trump has also maintained a disapproval rating of over 50% his whole term, something that Reagan and Clinton only managed briefly.
What matters isn't where he compares to those other guys right now. Sure, Reagan and Clinton got re-elected, but they also didn't have net negative approval their entire terms, either. In their third year, their net approval ratings ended up back in the positive.
No, what matters is how his approval rating is doing as we close in on Nov. 3, 2020. If Trump is still in the negative, he likely will not be re-elected. The only other one-term presidents we've had in modern history also happened to be the ones that had more people disapproving of them than approving. And in these highly partisan times, it's independents that hold the most sway in the approval ratings. According to Gallup, Trump has struggled to get much more than a third of independents to approve of his job performance. This is considerably worse than the last few presidents. Also, let's not forget about that Electoral College that most Republicans hated during the Obama era but now suddenly love. Trump's approval rating is significantly underwater in several major Rust Belt states that he picked up in 2016. If the Democrat can flip just Michigan, Wisconsin, and Pennsylvania (states Trump only barely won) back to blue, then they win. Iowa could flip. Ohio is once again pure toss-up. Arizona and possibly Georgia could become highly competitive. If the election were held today, Trump likely wouldn't win. But he has bit over two years to get his net approval into the positive, at least in the places that matter (and let's be honest: most of America doesn't matter in presidential elections). These past 19 months aren't exactly encouraging for that prospect.